@Old Salt Excellant Question. What MarciKS posted pretty much outlines the different types of diets and the some of the reasons people eat that way. I eat a vegan diet, I am not an ethical vegan. According to those who really need to care, I would be referred to as a strict vegetarian--means I don't consume animal products. I do it for health reasons, which if you're interested I'll go into detail.
The reason ethical vegans don't consume dairy is that cows are perpetually kept pregnant to produce milk. The male calves are separated from their mothers and often killed, both practices they deem as animal cruelty. Female calves are kept for milk production and the cycle starts all over again. Those who are vegan for the environment are also against livestock in general because it uses more resources and can cause environmental damage from converting forrest eco systems to agriculture to grow feed for animals, run off pollution, green house gas emissions, etc. They also eschew other animal by products , the most obvious being leather.
Eggs the issue is that male chicks are separated, destroyed, and often used as chicken feed-- keeping only the female chicks for laying stock. Also some of the conditions of commercial chicken farming are also considered quite cruel, like the conditions the animals are housed in.
I know there are a lot of stereotypes out there about vegans, but I'm the only vegan I know in real 3D life. I have an online friend who became plant based, but in fact eats vegan again for health reasons.
I'm not one of those people who are a vegan activist, or feel all that superior about my stance. In fact, the only people I care about what they eat are myself, my wife, and anyone we have invited over for a meal.
Both my wife's and my experience with the health benefits of changing our diets is enough reward. That and it makes washing up dishes, so much easier. I mean that quite literally. When I first decided to try eating this way it was a 2 week test, what kept me motivated really was how quick and easy clean up was. I'm someone that loves to cook, dishes-- not so much.