Whereas a "conventional" loaf of bread is usually made with only one or two types of flour - most often wheat and/or rye - gluten-free baking often uses a blend of flours. Blends using a number of different flours may be formulated to be "wheatlike" in flavor and aroma; however, this is not always the reason for a long ingredient list. Different flours have many different characteristics that affect their behavior, so sometimes a blend of many flours is chosen in order to combine these properties in a way that gives better results than when used alone. That said, sometimes only one or two flours are needed in a recipe.   

Here's a list of the ingredients you might find in my and others' gluten-free recipes, with a brief description of each one's characteristics and uses. For questions about ingredient substitutions, please see my FAQ page, and feel free to email me if you don't find an answer there.

Flours & Meals

Almond: Available as blanched (without skins) or natural (with skins - the whole almond). The choice is mostly aesthetic. The blanched kind is essential to the popular macaron cookie. Almond meal is also used in some traditional cookies and other sweet baking, such as sponge cakes, particularly in recipes from Italy and Germany. Used extensively in grain-free and low-carb baking, often in combination with egg whites.

Arrowroot: A very light, fine starch that is good for thickening - however, it does not work very well in milk-based sauces/puddings, where it develops an unpleasant stringy texture. Useful in baking where a "powdery" texture is desired, such as in shortbread or certain types of cookies. It is more neutral-tasting than cornstarch or potato starch, making it useful when starch is used uncooked, such as in powdered sugar.

Buckwheat: It's not remotely related to wheat; in fact, it's not even truly a cereal, as it is in a different plant family altogether. Nonetheless, it has many grainlike properties and is traditionally used as a grain, especially in Europe and Asia, in a variety of different baking and cooking applications, including porridges, pancakes, crepes, and cakes.
There are two main types of buckwheat flour: light and dark. They are made from the same thing; however, dark buckwheat flour contains some of the black hulls along with the grains themselves. Depending on how it's processed, this type can be unpleasantly bitter to some people. A very high-quality dark buckwheat flour is produced by Anson Mills.
Light buckwheat flour does not contain any of the hulls. It's usually not easily found in stores; the best way to get it is to simply grind it in a coffee grinder or food processor from whole, raw buckwheat.

Whole raw buckwheat: Bob's Red Mill
Dark buckwheat flour: Anson Mills; Arrowhead Mills

Buckwheat, malted: While "malt" usually means something off-limits for gluten-intolerant people, it actually refers to a process rather than any specific grain; essentially the grains are sprouted and then dried with heat. This gives a nutty, sweet flavor, and depending on how much heat is used to dry them, also contributes enzymes which break down starches into smaller carbohydrate components (high heat renders these enzymes inactive). Instructions on malting buckwheat are given in this post.

Chestnut: In contrast to other "nut" flours, which store a lot of energy in the form of fat, chestnut contains much of its energy in the form of carbohydrates. This means it behaves more like a starchy flour than like other nut flours. It is very finely-textured, starchy, and slightly sweet. It has traditionally been used for polenta and flatbread in some areas of Corsica, Italy, and southern France. It is also sometimes used in cakes and other sweets.

Chia: Light and dark chia are usually interchangeable for practical purposes. Personally, I find the dark variety to have a more pronounced nutty flavor, while the white seeds are more neutral and have a slightly thinner hull (making them less noticeably crunchy when used whole). Chia has excellent binding properties, forming a mucilage (gel) that is more complex than that of flax, psyllium, or common gums. It provides these properties when used whole as well as when ground into meal, though the whole seeds will need more time to fully hydrate.

Cornmeal: There are many different types of cornmeal. The two most common basic categories are white and yellow, with the latter usually having a more distinct corn flavor. Another variable is how finely it is ground: often plain cornmeal will have a wide variety of particle sizes, while polenta is a more uniformly medium size and fine cornmeal is smallest. The particle size will determine texture - fine will be more smooth and flourlike, while coarse will be more distinct in baking.

Cornmeal Masa: This traditional tortilla ingredient is prepared by cooking whole dried corn with calcium hydroxide (historically in the form of ash and various mineral sources). This process, called nixtamalization, makes niacin bioavailable. It also causes chemical changes to the proteins and pregelatinizes the starch, giving this ingredient very different properties than regular cornmeal.  

Flax: A small seed used in gluten-free and conventional baking alike, as it is often added whole to multigrain breads. It can be either brown or golden-colored. The seeds produce a mucilage (gel), useful for binding, and as such are also often used as egg replacers in vegan baking.

Garbanzo Bean (Chickpea): This flour has an important place in many types of doughs and batters. It can have a smoother texture than some grain flours, and is useful for balancing and rounding out the flavor of starchy flour blends. Some people consider it bitter or overly "beany"-tasting; in small quantities, though, that taste does not come through, and in larger amounts I think it is all a matter of using it for the right things. For instance, there is a wonderful traditional type of flatbread made in the Provence region of France, and another almost identical bread in areas of Italy, that use this flour exclusively. It's also used extensively in Indian baking for a wide range of things, both savory and sweet.

Millet: This small, round cereal grain has many traditional uses, including cooking whole or as porridge in addition to grinding into flour. It's best in small amounts for yeast breads - more makes the bread crumbly, similar to cornmeal - but it can be used liberally in muffins and cakes. It is useful for its flavor and color when converting wheat-based recipes which call for semolina flour, such as fresh pasta and certain Italian breads. Millet absorbs more water than the same quantity of many other grains like brown rice, and will be gritty if insufficiently hydrated.

Oats: An estimated ~10% of celiacs also react to oats, so make sure it is OK for you to consume oats before using them.
The most significant property of oats from a baking perspective is their ability to form a sticky, pasty, gel-like consistency that can contribute to the texture and consistency of doughs and help bind liquids. It is not a strong enough effect to consider it a binder on its own.
Oat flour and/or rolled oats provide good texture in yeast breads, and are a traditional part of many soda breads, quick breads, and cookies. Steel-cut oats are a good chewy addition to multigrain breads and soda breads. Using oat flour in large quantities can create a grayish color in baked goods and result in a pale-colored crust.

Pea: It's green! Don't worry, the color is not noticeable when used in combination with other flours. Uses are very similar to garbanzo flour, though pea provides a lighter, sweeter taste.

Potato flour: Not to be confused with potato starch. Potato flour is made from finely ground dried potatoes. It should be used in small quantities to contribute a dense, moist texture.

Potato Starch: Not to be confused with potato flour. Potato starch is an isolated component, more processed than potato flour. It is a fairly neutral-tasting ingredient, and thus can be useful when a white/neutral flour is desired. Potato starch absorbs more water than the same mass of some other starches, so be sure to adjust your recipe accordingly. 

Rice, white: This is one of the most widely known flours in wheat-free baking. It can be used in just about anything thanks to its neutral flavor, though if large quantities are used, it can make baked goods too starchy and crumbly. It is traditionally used in tempura batter and other applications, as its small starch granules help provide a delicate, crisp texture. Rice is also frequently used in Indian steamed breads.
Rice flour has historically been used in some European and early American baking for certain cookies, cakes, etc., although some of these recipes will not work well with modern rice flour.

Rice, brown: This flour is made from the whole rice grain, as opposed to white rice, which has been polished to remove the outer layer. Compared to other whole grains it is also fairly light-colored and mild, so it can be used in most baking - in fact, I usually prefer it to white rice. Some scientific studies indicate it is better for texture and keeping qualities compared to white rice, possibly due to properties of the fiber and oils present. Large amounts of this flour can make your bread taste distinctly like rice, though, so it's best used in combination with other grains.

Rice, sweet (also known as mochiko): This type of rice has short, sticky grains, and is traditionally used in Japan to make very gummy little rice cakes called mochi - as well as being used in a lot of other Asian sweets. You might see it in recipes for cake, muffins, or pancakes, and for pate a choux. I find it useful for thickening sauces and soups as well.
Most or all of its starch is in the form of amylopectin, a more highly-branched structure than amylose, and thus is more resistant to retrogradation (recrystallizing) of the starch granules, which causes us to perceive a "dry" texture in baked goods. 

Sorghum: This is a round grain which is larger than its relative millet but smaller than rice. It can be cooked whole for pilafs and porridge. As a flour, it is slightly sweet, and as part of a flour blend it is usually more subtle in taste and texture than millet or cornmeal.

Tapioca Starch (also called tapioca flour): This is common in most gluten-free baked goods, and it is the primary starch I use. It is more finely-textured than potato starch. 

Tapioca Starch, modified: This is marketed under the brand name Expandex. You will see it in some of my older recipes, which also contain gums and egg - in some of these applications, it improves the rise and texture. However, in eggless yeast breads, and in things which do not contain gums, it does not seem to make a difference (with my particular formulas, anyway). I no longer use it.

Teff: These extremely tiny grains, which can be dark brown or, less commonly, ivory, are traditionally ground into flour for the Ethiopian sourdough injera. The dark kind has a slightly sweet, almost molasses-like scent when cooked. The grains are so small that you can even use them whole in bread, where they create an interesting texture and appearance.

Other Ingredients

Egg Replacer: Though it is often used to make recipes without eggs, it also may show up in addition to eggs in some gluten-free recipes. This is because it helps with binding and gives a little extra leavening - it is usually a combination of gums (often including modified cellulose such as carboxymethylcellulose aka 'cellulose gum'), baking-powder-like leavening agents, and starch. Some of my earliest recipes use it for these purposes. However, I now prefer to use other ingredients to achieve these results.

Guar Gum: A binding agent derived from locust bean (carob). It is not as potent as xanthan gum for thickening liquids. More highly-branched structure than xanthan gum. It shows synergistic interaction with some other binders, including xanthan gum, so as part of various combinations it may have different effects than when used alone.

Pectin: Pectin is a very good binding agent, and may help delay starch retrogradation (staling/hardening) in baked goods - I prefer it to the common types of gums. It is most frequently used in making jam - it is what helps it gel. Most brands of pectin contain extra ingredients such as cornstarch, dextrose, and citric acid. For pure pectin, see below.

Pomona's Pure Citrus Pectin: Unlike other brands of pectin, there are no fillers or extra ingredients added. (For this reason, if you are substituting a starch-containing pectin, use about twice as much as the amount of Pomona's specified in my recipe.) This is a special variety of natural pectin (some structural and chemical differences to the sort of pectin used in conventional brands), and I find it to be a more effective binder than the type of pectin used in other brands. Pomona's comes in a box with two packets: the large packet is pure pectin, the small packet contains calcium phosphate (a naturally occurring compound, which interacts with the pectin to help it bind). I recommend combining the contents of both packets in a small jar, and just measure from the jar whenever a recipe calls for Pomona's.

Psyllium Husks: The husks of small seeds, which I use as a binding agent. They interact with water to form a gel much like flax seed powder, but unlike flax, they are soft and barely noticeable in the baked food. I generally use the intact husks in breads, and powdered husks in other things. Psyllium has a subtly sweet/grainy aroma.

Xanthan Gum: A binding agent made by microorganisms, found in most pre-made GF products and baking mixes, and in many GF recipes. It's a very potent binder, meaning a very small quantity can thicken and stabilize a relatively large volume of liquid, and unlike some binders it does this at room temperature. These properties mean it is widely used in the food industry for a large variety of applications. It also shows synergistic interaction with some other binders, including guar gum, so as part of various combinations it may have different effects than when used alone. In GF products, it can also have the effect of keeping the product feeling soft and moist (there are many other ways to accomplish this, though).
Because it is so potent, formulas which use too much of this binder can easily have a noticeable pasty mouthfeel. In my opinion, it promotes a crumb structure which is not the best suited for bread making, and can cause doughs to be more frustrating to work with due to its particular viscous properties. I no longer use it.