Vegetarianism (ie. not veganism) encompasses the practice of following plant-based diets (fruits, vegetables, etc.), with or without the inclusion of dairy products or eggs, and with the exclusion of meat (red meat, poultry, and seafood). Abstention from by-products of animal slaughter, such as animal-derived rennet and gelatin, may also be practiced.
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.
* Ovo vegetarianism includes eggs but not dairy products.
* Lacto vegetarianism includes dairy products but not eggs.
* Ovo-lacto vegetarianism (or lacto-ovo vegetarianism) includes animal/dairy products such as eggs, milk, and honey.
* Sentient vegetarianism (also known as yogic or sattvic diet), a plant based diet which may also include dairy (not eggs) and honey, but excludes anything from the onion or leek family, red lentils, durian fruit, mushrooms, blue cheeses, fermented foods or sauces, alcoholic drinks and often also excludes coffee, black or green tea, chocolate, nutmeg or any other type of stimulant such as excess sharp spices.
* Buddhist vegetarianism (also known as su vegetarianism) excludes all animal products as well as vegetables in the allium family (which have the characteristic aroma of onion and garlic): onion, garlic, scallions, leeks, or shallots.
* Jain vegetarianism includes dairy but excludes eggs and honey, as well as root vegetables.
* Macrobiotic diets consist mostly of whole grains and beans.
This is also interesting:
The body can preserve stores of B12 for up to 30 years, and reuses the vitamin without destroying the substance. Clinical evidence has shown a deficiency of B12 in vegans and, to lesser degree, vegetarians.
So, when you start a vegan diet at around 30, then after 20 or 30 years you will get into problems because of a B!2 deficiency.
There are vegetarian sources of B12.