You're venturing into vegetables. Nice.

Tofu is a curd made from soy milk. The process is similar to making cheese out of regular milk. This stuff is very versatile; you can find it in a range of densities from silken (great as a substitute for cream when cooking) to extra firm (for grilling and stir-frying).

Tempeh (tem-PAY) is also soy, but it's a whole-bean cake that is much denser than even the firmest tofu. This density gives it a nougat-like, satisfying texture. Because it includes the entire bean, Tempeh is really high in fiber.

Seitan (SAY-tahn) is wheat gluten, or wheat protein that is separated out from the rest of the grain. Seitan has a fleshy texture almost like chicken and is fat-free.

Because of their high protein content, tofu, tempeh, and seitan are often referred to in the western world as meat substitutes. If you're like me though, nothing will ever replace meat.
Try not to think of these foods as "substitutes" when you are trying them for the first time or you will most probably be let down. Like anything else, they are what they are, delicious in their own right when prepared well. Appreciate them just for that and you'll enjoy a little extra heart health while you're at it.