A vegan (pronounced VEE-gun) is a vegetarian who, in addition to not eating animal products, assumes a cruelty-free lifestyle.  This includes not using any materials derived from animals, like furs, leathers, etc.,  along with avoiding items that use animal byproducts or are tested on animals.

The three reasons to consider veganism:

1)  For the animals:  Modern farming practices inflict extreme cruelty, torture and suffering on farm animals who are harvested for flesh, milk, eggs, etc. (a) See how modern humanity treats animals:  All-Creatures.org

2)  For the environment:  Animal agriculture exerts extreme stress on the environment.  The waste produced from these farms is a major source of pollutants.  Also, producing animals for consumption is an inefficient process.  The land, resources, effort, etc., that go into animal farming could produce much more human food if done directly.  For example, it takes 16 pounds of grain (b) and  2500-6000 pounds of water (c) to produce one pound of beef ,  (A pound of wheat can be grown with 60 lbs of water)

(3)  For your health:  The consumption of animal products is linked to heart disease, colon cancer, osteoporosis, kidney disease, and other illnesses.(d)  A vegetarian diet has been proven to reverse heart disease.(e)

(a) Factory Farms
(b) The environmental argument
(c) For the planet
(d) Physician's Committee for Responsible Medicine
(e) Reversing Heart Disease with a vegetarian diet

For more information read an Introduction to Veganism