Meat is an excellent source of protein, vitamin B12, carnitine, creatine and iron.
Consumption, and especially overconsumption of red meat is associated with increased risks of colorectal cancer, some other cancers and heart disease.

Not all meat is the same: there are lean meats and fat meats. You can’t put chicken breasts in the same category with pork or beef.
There is some concern about hormones and antibiotics present in meat, however, those are generally trace amounts and do not endanger health, unless meat is overconsumed.
More realistic concern is saturated fat content. So if you eat meat, try to choose less fatty meats, such as chicken, turkey and veal, instead of pork. Generally try to be mindful about your meat consumption, especially red meat consumption.

When it comes to beef, it’s a typical red meat. If you eat it, limit consumption to maximum once a weak. It can be good to eat it sometimes, because it contains a lot of iron and carnitine. In some countries there is maybe still risk of mad cow disease, so the best advice would be to AVOID COMPLETELY CONSUMPTION OF BRAIN OR OTHER NERVOUS TISSUES AND BONE MARROW, and if you eat beef, don’t eat meat of older animals, better young beef and veal. Also, only eat beef if you are well informed about the origin, safety and quality of meat.

Pork is generally safe if it’s well cooked. Undercooked pork can infect you with trichinella. Also eat pork only from well known sources, and meat which is safe and tested. NOT ALL PORK IS TOO FATTY. Pork, even if considered a fatty meat, from the point of view of safety, and cancer risk as well, is better than beef.

Generally, your safest bet is poultry, young veal and beef from well known sources, and pork which is not too fatty. Sometimes, you can eat lamb as well, in some countries eating lamb is done only in special occasions and to celebrate something.

A very important thing to add: by meat should be considered just that – plain meat. Not industrially (over)processed meat products, such as salami, sausages, hot dogs, etc… Such foods, while very convenient at times, are quite risky, due to their elevated levels of preservatives, especially sodium nitrate, which is known to increase the risk of colorectal cancer. Such industrial products should be viewed as a separate category, very similar to junk food / fast food, and while they can be a convenient way to consume some meat, they should by no means be your main forms of meat intake.

Finally, if you are a vegan or vegetarian, you don’t have to eat meat, but you must try to get all the protein from combining various plant sources (since plant proteins are incomplete, you have to combine more different plant proteins to make them complete). Also vitamin B12 is present exclusively in meat, fish, eggs and generally animal products, and if you don’t eat any of those things you MUST TAKE B12 SUPPLEMENTS, OR EAT B12 FORTIFIED FOOD.