What makes something Kosher (in general)?

Only certain animals can be eaten. Of the specific animals that can be eaten, the birds  and mammals must be killed in accordance with Jewish law. All blood must be drained  from meat and poultry or boiled out of it before it is eaten. Certain parts of permitted  animals may not be eaten. The meat of poultry or mammals cannot be eaten with  dairy. Fish, eggs, fruits, vegetable and grains can be eaten with either meat or dairy.  Some believe that fish may not be eaten with meat. Cooking utensils that have come  into contact with meat may not be used with dairy and vice versa. Utensils that have  come into contact with non-kosher foods may not be used with kosher food. 

What animals can I eat? 

According to Lev. 11:3 and Deut. 14:6 an animal that has cloven hooves and chews it  cud are permitted to be eaten. Any animal that does not have both of these qualities  is forbidden. The camel, badger, hare, and pig are not kosher because each lacks one  of these two qualifications. Sheep, cattle, goats, deer and bison are kosher as they  have both cloven hooves and chew their cud.   The mammals that may be eaten must be slaughtered in accordance with Jewish law  Deut. 12:21. We may not eat animals that died of natural causes or that were killed  by other animals. The animal must have no disease or flaws in the organs at the time  of slaughter. The method of slaughter is a quick, deep stroke across the throat with a  perfectly sharp blade with no nicks or unevenness. This method is painless, causes  unconsciousness within one or two seconds, and is recognized as the most humane  method of slaughter possible.               

Blood Drainage

Jews are forbidden to consume blood Lev. 7:26-27 and Lev. 17:10-14 as it is  explained that blood is the life of the animal. This applies only to blood of mammals  and birds, not fish.   The first step to removing the blood happens during slaughter. The blood remaining  must be removed by broiling or soaking and salting. Liver may only be kashered by  the broiling method, because it contains so much blood in it. This final blood removal  must be completed within 72 hours after slaughter and before the meat is frozen or  ground. Most butchers and kosher meat packing facilities will take care of the soaking  and salting for you, but always check this when you are buying from a new butcher.