The choices are usually in one of these categories:
Vegan: No meat, of course, of any kind, and no animal bi-products. That includes no wearing or owning leather, no eggs, no dairy, and a little known one, no jell-o. Jell-o is made of gelatin, which is basically crystallized animal fat. Ew, right?
Lacto-Vegetarian: Nothing above, but you can drink milk and eat cheese and cheese cake and ranch dressing and certain yogurts, and ice cream. Part of the reason some people are vegan is that some dairies, even organic ones, still use male dairy calf babies for veal. For more information, just Google it. You could've just Googled this, but then again you might have.
Ovo-Lacto Vegetarian: Eggs and milk, still NO MEAT! AT ALL! Thus the term, VEGEtarian. Egg laying hens are often kept in tiny cages with bad conditions. You have to choose eggs carefully, there are cage free, and vegetarian raised, though how do they control that if they're cage free? Any ideas? This isn't a problem for me because my family owns chickens and so do many of our close personal friends. So this is my choice.
Also, this category is only sort of a vegetarian; Pescetarian: No red meat, and maybe no poultry. Fish and/or shellfish is allowed.
I hope this was at least a little bit helpful,