Followers of this particular diet are to eat only food that it doesn't come from animals. Not only you can't eat meat, but anything that come from animals such as milk or eggs, for example.
Vegans don't want to hurt the animal nature in any way. They don't even wear any clothes that could come from animals, like leather or fur.
They lack a lot of properties and quality substances that only provide animals, like B12 vitamine, but I think to have peace in their conscience compensate this lack. Of course, experts in food, nutritionist and other scientist advise against this radical diet that practise a minority. We can find some famous people who are vegans, for example Uma Thurman or Michelle Pfeiffer.