Vegetarianism can be adopted for different reasons: In addition to ethical reasons, motivations for vegetarianism include health, religious, political, environmental, cultural, aesthetic or economic. There are varieties of the diet as well: an ovo-vegetarian diet includes eggs but not dairy products, a lacto-vegetarian diet includes dairy products but not eggs, and an ovo-lacto vegetarian diet includes both eggs and dairy products. A vegan diet excludes all animal products, including eggs, dairy, and honey.

Various packaged or processed foods, including cake, chocolate, and marshmallows, often contain unfamiliar animal ingredients, and may be a special concern for vegetarians due to the likelihood of such additions. Vegetarians vary in their feelings regarding these ingredients, however. For example, while some vegetarians may be unaware of animal-derived rennet's role in the usual production of cheese and may therefore unknowingly consume the product, other vegetarians may not be bothered by its consumption. Often, products are scrutinized by vegetarians for animal-derived ingredients prior to purchase or consumption. The results of a recent International survey suggest the standard definition of vegetarianism is different in different nations. Vegetarians in some nations consume more animal products than those in others.

Semi-vegetarian diets consist largely of vegetarian foods, but may include fish or poultry, or sometimes other meats on an infrequent basis. Those with diets containing fish or poultry may define "meat" only as mammalian flesh and may identify with vegetarianism. A pescetarian diet, for example, includes "fish but no meat". The common use association between such diets and vegetarianism has led vegetarian groups such as the Vegetarian Society to state that diets containing these ingredients are not vegetarian, due to fish and birds being animals.