What makes food kosher?!
Ok - let's first address a common misconception; a Rabbi does NOT bless the food. Rather the food is prepared under rabbinical supervision so that none of the laws are violated with preparation, cooking, etc.
Meat must be slaughtered in such a way as to not cause pain (a bolt in the head of a cow is not kosher) and no blood can be used/eaten. Milk and meat cannot be prepared together nor can they share a plate (which is why those that keep kosher have separate dishes, silverware/utensils, etc.)
Animals must 'chew their cud' AND be 'cloven/split of hoof' in order to be a kosher animal, while fish must have scales and gills to be kosher.
Check out: http://www.jewfaq.org/kashrut.htm for a great explanation.
Reconstructionist Jew - raised Conservative
The way it's slaughtered/prepared.
Being blessed by a rabbi, that it doesn't break any of God's dietary laws, according to Jewish custom. Also it must not be contaminated with anything else then Kosher food.
No pork, shellfish, and they don't mix dairy in meat.
Hopefully this helps :)
Being blessed by a Rabbi