A vegan diet is a form of vegetarianism which excludes all animal products from the diet, such as meat, poultry, fish, dairy products, eggs and honey. Strict veganism also excludes the use of animal products such as wool, silk, leather, and fur for attire and adornment, even though some of these do not directly involve the death or slaughter of an animal.

Most vegetarians consume dairy products, and many eat eggs. Lacto-vegetarianism includes dairy products but excludes eggs, ovo-vegetarianism includes eggs but not dairy, and lacto-ovo-vegetarianism includes both eggs and dairy products.

Semi-vegetarianism consists of a diet largely of vegetarian foods, but may include fish and sometimes even poultry, as well as dairy products and eggs. The association of semi-vegetarianism with vegetarianism in popular vernacular, particularly pescetarianism (also called pesco-vegetarianism and described as a "vegetarian" diet that includes fish defines "vegetarian" (noun) as "A person who on principle abstains from animal food; esp. one who avoids meat but will consume dairy produce and eggs and sometimes also fish (cf. VEGAN noun)."

The reasons for choosing vegetarianism may be related to morality, religion, culture, ethics, aesthetics, environment, society, economy, politics, taste, or health. A generic term for both vegetarianism and veganism, as well as for similar diets, is "plant-based diets". Properly planned vegetarian diets have been found to satisfy the nutritional needs for all stages of life, and large-scale studies have shown vegetarianism to significantly lower risks of cancer, ischaemic heart disease, and other diseases.

Other dietary practices commonly associated with vegetarianism

• Fruitarianism is a diet of only fruit, nuts, seeds, and other plant matter that can be gathered without harming the plant.
• Su vegetarianism originating in Buddhism, excludes all animal products as well as the fetid vegetables: onion, garlic, scallions, leeks, or shallots.
• Macrobiotic diet is a diet of mostly whole grains and beans. Not all macrobiotics are vegetarians, as some consume fish.
• Raw veganism is a diet of fresh and uncooked fruit, nuts, seeds, and vegetables.
• Dietary veganism: whereas vegans do not use animal products of any kind, dietary vegans restrict their veganism to their diet.

Some vegetarians also avoid products that may use animal ingredients not included in their labels or which use animal products in their manufacturing i.e. cheeses that use animal rennet, gelatin (from animal skin, bones, and connective tissue), some sugars that are whitened with bone char (e.g. cane sugar, but not beet sugar) and alcohol clarified with gelatin or crushed shellfish and sturgeon. Vegetarians who eat eggs sometimes prefer free-range eggs (as opposed to battery farmed eggs) on moral grounds.