There are vegetarians who eat no animal products at all (even eggs and milk), and these people are 'vegans'. There are those who do eat cheese, milk and eggs, and these people are 'vegetarians'. Then there are those vegetarians who also eat fish, and some who will even eat chicken or meat on rare occasions. My wife and I have eaten no red meat or poultry for years (except on rare occasions at family gatherings where no other options are available), but do eat fish and shellfish regularly.
There are many reasons I choose to be a vegetarian: health reasons, environmental reasons and reasons of conscience. Most vegetarian diets are considerably healthier than non-vegetarian diets. Though one could eat only ice cream, chips, soda, candy bars, donuts and white bread and still be a vegetarian. However, studies that are available suggest the vegetarians who eat fruits, vegetables, whole grain carbohydrates, nuts, legumes and plant oils tend to live a few years longer than nonvegetarians, tend to weigh a few pounds less than nonvegetarians, and tend to have lower cholesterols than nonvegetarians. I believe that the best diet for good health is a variation on the so-called 'Meditarranean diet', in which most of the calories come from typical vegetarian choices (whole grain carbohydrates, legumes, fruits, vegetables, nuts), but also includes fish or shellfish a few times per week. There is no reason to eat meat, pork or poultry in such a diet, and I choose not to.
Environmental reasons also play a large part in my decision to forego meat and poultry. The amount of resources (gasoline, water, fertilizer) required to grow 1 pound of fruits and vegetables is far, far smaller than that necessary to grow 1 pound of beef or chicken. It takes 1600 calories from oil, gas and other fossil fuels to produce 100 calories' worth of grain-fed beef. It takes 500 calories from fossil fuels to produce 100 calories' worth of chicken. But it takes only 50 calories to produce 100 calories' worth of plant foods. It takes about 18,000 gallons (yes 18,000 gallons) or rain and irrigation water to produce a pound of beef - but far less to produce a pound of vegetables or fruit. And what about all the manure from cattle and pigs, as well as chicken waste from huge commercial chicken production facilities - this waste (as well as run-off from ferilizer used to grow the grain to feed the animals) is a major cause of pollution in the Chesapeake Bay, as well as southern and western states. The methane from cattle is 23-times more potent as carbon dioxide in its contribution to global warming.
My last important reason for being a vegetarian is that of conscience. The conditions in which cattle, pigs and chickens are raised in this country is appalling. Cattle at least spend some of their lives grazing. The last 2 months are spent on feedlots eating a high grain diet that frequently causes stomach ulcers, liver abscesses and hoof disease. Pigs are raised in huge sheds, surrounded by manure and urine - they never get a chance to walk around in the sun or rain or root for food. Chickens are raised in huge sheds housing up to 50,000 birds. Their beaks are trimmed to keep them from pecking each other, even though 'debeaking' causes pain that recurs with eating. These chickens never get a chance to build nests, peck for bugs or walk in the sunshine. So-called 'free range' chickens are required by the USDA to have access to the outdoors for all of 5 minutes per day.
A typical modern slaughterhouse will kill 13,000 chickens per hour, 1000 pigs per hour or 250 cattle per hour. At those rates, is it likely impossible to assure that every animal is adequately stunned before it is killed. Chickens are stunned by placing their heads in electrified water, after which their necks are slit. Livestock are crammed into crowded trucks lacking food and water; some are still conscious when they are slaughtered, resulting in horrible deaths, and putting slaughterhouse workers at risk of injury. Sweden, Denmark and some other European countries have adopted much more humane animal husbandry rules, such as requiring sedatives/anesthetics prior to dehorning cattle (not required in this country), requiring straw bedding for pigs, and requiring more cage space per chicken (in the US, each chicken has about a half a square foot of living space).
Most of my day is spent counseling people about diet and health. Diabetes is now an epidemic, as America gets fatter and fatter. I doubt that most Americans would even consider becoming vegetarian, but I hope that I can convince you to cut down on animal products and try to eat more fruits, vegetables, nuts and legumes (red beans, black beans, split peas, lentils, chickpeas). You will be helping yourself as well as your environment.