What are those animals? Cattle are required to have split hooves and chew their cud. Animals that fit this description include cows and lambs. Out at this stage are camels, hares, and (shocker) pigs. Fish are required to have fins and scales, and only the birds listed in the text are allowed. Bugs and shellfish are prohibited.
These are the very basic kosher ground rules. As I mentioned, however, keeping kosher entails more than just avoiding certain combinations and animals. There is a great concern in the legal literature about contamination and supervision. If I boil milk in a pot previously used to make beef stew, this constitutes a prohibited mixture, even though both meat and milk aren't technically in the pot at the same time. Any item used for food preparation is considered to have a kosher (or nonkosher) status based on the food it has been used to prepare.
Given the great amount of knowledge required to keep kosher, there is a similar concern with food prepared by those who don't know or follow the rules. Can I assume that the vegetarian entree served to me at a local restaurant truly contains no meat byproducts, and that the pans and utensils are used only for vegetarian dishes? The same suspicion holds true for manufactured food products: without proper supervision, can a consumer be assured that no trace elements of the prohibited foodstuffs have made their way into food?
Food is therefore broken into three categories:
Meat (Hebrew: b'sari, Yiddish: fleishik/g/kh)-- contains meat in some form, or cooked in a pot recently used to cook meat.
Pareve-- contains neither dairy nor meat.
Congratulations! You now know the basic laws, what can and can't be eaten and the 3 denominations of food. Next time I'll talk about the basics of keeping a kosher kitchen: just how many sets of pots and pans do you need? How about dishes? How can you tell if a food item is kosher? How about a restaurant?
As always, please feel free to ask anything, either through comments, email or formspring.