Textured vegetable protein (TVP): While made of defatted soybeans, it is not tofu. TVP provides a meat-like taste and texture and can substitute for ground meat in a variety of dishes.
Seitan (wheat gluten): Gluten is the protein in wheat; seitan is made by soaking high gluten wheat flour with water and kneading the mixture until it starts to solidify. Seitan is chewy and bland by itself, which is why it is especially well-suited for spicy or other strongly seasoned dishes. Seitan can be purchased ready-made or in easy-to-make packaged mixes.
Tempeh: Made from fermented soybeans, tempeh has a distinct flavor and meaty texture, which can be used in place of ground meat, and works well in curries and stir-fries. Because tempeh is made from whole beans, it is a good source of fiber.
Tofu: A curd made from soybeans, tofu is mild in taste and easily absorbs the flavor of any recipe. Soft tofu works well in soups, sauces, and desserts, and firmer varieties work well in stir-fries and other recipes requiring tofu to hold its shape. For a denser texture, freeze tofu, thaw, and squeeze out excess water before using it in your recipe.
Sources: Physicians for Responsible Medicine, "Restaurant Vegetarian Starter Kit," available at http://www.pcrm.org/health/RVSK/RVSK7.html; Parker, Amy, "Vegetarian Cuisine," About.com, available at http://vegetarian.about.com/od/foodsingredients/g/
seitan.htm; "Tempeh," Wikipedia, available at http://en.wikipedia.org/wiki/Tempeh.