From Envirowiki

Veganism is a lifestyle in which one does not use any animal products. This includes meat, dairy, eggs, leather and other products.

Reasons for veganism include compassion for animals who are regularly and sometimes severely mistreated, health reasons, and environmentalism.

[edit] Veganism for the environment

Animal products, as a general rule, require more energy (food) input, and more water input than their similar plant based counterparts. This is especially true for meat, by a large factor, but is less true, or possibly untrue^{[reference needed]}, for eggs (as opposed to e.g. tofu, for protein), or wool (as opposed to cotton), or in other situations where the animal product is produced by the animal multiple times, and taking it does not kill the animal.

there is an extremely in depth article on this topic (actually environmental vegetarianism) at wikipedia.org.

[edit] sources

• Diet, Energy and Global Warming, Gidon Eshel1 and Pamela Martin, Dept. of the Geophysical Sciences, The University of Chicago