Vitamin B12 is an essential vitamin that is important for proper functioning of many organs in the body. Principally, B12 deficiency can lead to a form of anemia
and problems with nerve function.
B12 is found only in foods that are of animal origin. For this reason, people who follow a vegan diet (no foods of animal origin at all) are at risk of developing a B12 deficiency. In reality, many vegans do obtain enough B12; they do this by eating a variety of fortified foods (for example, many cereals and grains that are commercially available are now fortified with B vitamins).
On the hand, less strict vegetarians, who eat eggs or milk or cheese, for example, will not be in any danger of a B12 deficiency, as all of these products have sufficient quantities of B12.
There are some who claim that B12 can be found in certain seaweed and other products and that this may serve as an alternative B12 source for strict vegetarians. However, most of the B12 that is found in strictly plant sources is bound tightly to other substances and is not easily absorbed by the body.
If you are a strict vegetarian, you should be able to get sufficient B12 just by eating a daily serving or two of the fortified cereals or other products (just read the labels).