Vegetarian, vegan, macrobiotic, pescaterian...

 There are different alternative ways to choose our food.

A vegetarian doesnt eat meat, fish , poultry or diary foods.

A lacto-ovo-vegetarian is a vegetarian who does not eat meat, poultry, fish, shellfish or animal flesh of any kind, but is willing to consume dairy and egg products.

A vegan doesn't eat / use products of animal source, so no meat, fish, poultry, diary products, honey etc..and does not wear leather.

A macrobiotic  avoids refined and unnatural processed products, and most macrobiotics also avoid meat, sugar , coffee , diary and non-organic products. Macrobiotics state there is an order in this Universe, and if we find and follow this order, we will get health, happiness and longevity. Most of macrobiotic diets are centered around whole grains. Macrobiotics is a "new" way of understanding the teachings of  "The Yellow Emperor's Classic of Medicine".

A pescaterian is like a vegetarian, but he eat seafood too.

The reasons for choosing vegetarianism/veganism may be related to morality, religion, culture, ethics, aesthetics, environment, society, economy, politics, taste, or health.
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,  heart disease, and other diseases. 
The reason for choosing macrobiotics is related to mastering  health and for some, also to the philosophy behind macrobiotics.
Of course there are also mixes of the above 4 classes, and there are also other classes, mostly related to religion.