Animals that live on the land must chew the cud and have a cloven hoof. This means that cattle, sheep, deer and goat are kosher, whereas as pigs and horses are treif. An animal which crawls on the Earth, for example a snake, is also unkosher, as are carrion.
The animals must be killed in accordance to the laws of Shechita, hence a sheep killed by any other method is unkosher.
Meat must also be kashered before eaten - a cleaning process in the home to remove it of excess blood. Blood is unkosher because it symbolises life. The kashering process involves soaking and salting the meat.