If you buy a piece of meat in a kosher supermarket, what exactly is it that makes it kosher? Or the other way around, what is wrong with a normal piece of meat I buy at a random supermarket?
Basically there are four things that need to be (properly) done to any piece of meat to make it kosher:
- The meat must come from a kosher animal.
Basically, a land animal has to have split hooves and chew the cud (see this post for details), there’s a limited list of allowed birds (see this post for details). Meat from a non-kosher animal, e.g., pork, can never be kosher, no matter what you do.
- The animal must be slaughtered properly.
It is forbidden by Torah to consume blood (Leviticus 7:26). The main goal of Jewish slaughter (shechita in Hebrew) is to get as much blood out as possible, while being as fast as possible in order to cause as little pain as possible. This is accomplished by cutting the throat in one quick stroke with a very sharp knife. Also the internal organs are inspected for any things that may make it nonkosher. There are lots of specific rules and the slaughtering has to be performed by a trained professional.
- The correct parts of the animal have to be taken.
The sciatic nerve and specific fats around the vital organs may not be eaten and have to be removed. Again, the rules are very specific and you need to be trained to do this right.
- The meat must be soaked in water and salted.
Again, the goal is to get the blood out. The meat is soaked in water for more than 30 minutes, then salted and left to lie in salt for an hour, then rinsed and washed.
At least the first three steps and most of the time also the last step is done at the kosher butcher, so you need not worry about it too much from a practical point of view. If you are ever in the situation where you have to soak and salt your meat, refer to a book or a competent rabbi for the details (e.g. there are a few pages about it in "The practical guide to Kashrus" by R. Shaul Wagschal).
Note that the above rules apply to land animals and fowl, they do not apply to fish. Fish have to have fins and scales to be kosher, but they do not have to be drained of blood and do not have to be slaughtered in a specific way. Any part of fish can be eaten.
I hope this clarifies a bit what distinguishes a kosher piece of beef from a non-kosher one. The whole process is done by a person trained in slaughtering and the relevant rules who is called shochet. There is no rabbi or clerical person needed. No "blessing of the meat", mystical ceremony or anything is done. Jewish slaughter is a traditional way of getting the blood out.