Vegetarians eat mainly fruit, vegetables and cereals. There are far more vegetarians in the world than meat eaters. This is simply because vegetables, grains and the like are easier to keep without refrigeration and are usually more readily available. Though for many generations most of the developed world have been enthusiastic consumers of meat. To a degree this is changing, for reasons involving health, religion, environmental concerns and trends.
There are several types of vegetarian diet. The most common form is lacto-ovo-vegetarianism in which milk and eggs are consumed, but no flesh. Lacto-vegetarians eat dairy products but not eggs. Other vegetarians avoid red meat but eat fish. A vegan diet excludes all animal products (not only meat and fish but also dairy produce and eggs).
There is no reason why a vegetarian diet cannot be as healthy as a diet containing meat, provided that protein is obtained from nuts, cereals or pulses (eg. beans). Vegetarians also need to ensure they get adequate supplies of iron, zinc and calcium, which are found in good supply in meat and milk. Women in particular have double the iron needs of men and should take care to avoid iron deficiency. Women also seem to suffer more from loss of calcium. Generally an adequate supply of these minerals can be obtained from dairy products, but if these are not included in the diet, substitutes must be found. A vegan diet is likely to be deficient in vitamin B12, and supplements may need to be taken to avoid this.