Veganism involves not eating any animal derivatives. As a family, we are not strictly vegan (but nearly so), since we do eat our own very ‘free-range’ and very ‘organic’ eggs. The chickens run free, when possible and join in activities on the premises.
Dogs can apparently become vegan, since their nutrient requirements are so similar to ours. They do not, however, have the correct dentition for this, so food preparation must take account of this, or malnutrition will result. In particular, their teeth and gums may suffer without raw bones to chew. The AVMC does not recommend making a dog vegan but works with patients that are, being careful not to prescribe medicines that have been derived from animal material.
Cats are unable to be vegan or even vegetarian. Their dietary needs cannot be supplied from solely non-animal sources.
Veganism is often a philosophical statement. Vegans strive to prevent the suffering of animals, involved not only with meat production but also for egg and milk production. It is not always understood that, in order for cows to produce milk (including all dairy products), their calves must be taken away. This in itself is a source of some suffering. There is the unfortunate aspect that some of the calves have no value so are killed, while others not suitable for future breeding are put into meat production.
See also: EarthSave Report on relationship between Vegetarianism/Veganism and reduction in greenhouse gases: http://www.earthsave.org/news/earthsave_global_warming_report.pdf
“People should have one meat-free day a week if they want to make a personal and effective sacrifice that would help tackle climate change, the world's leading authority on global warming has told The Observer. Dr Rajendra Pachauri, chair of the United Nations Intergovernmental Panel on Climate Change, which last year earned a joint share of the Nobel Peace Prize, said that people should then go on to reduce their meat consumption even further.” (click).
“Has it been Proven that a Vegetarian Diet is Really Healthier? The short answer is a resounding yes; it has been conclusively proven through extensive worldwide studies by independent, highly respected international health advisory boards that a vegetarian diet is significantly healthier than one which includes meat and animal products. This is true for all ages, infant to adult, and includes pregnant and lactating women.” (click)