Shechitah

Determining that an animal is of a kosher species is just the first step. Further laws govern how the animal should be killed and which parts of it can be eaten

Kosher mammals and birds are slaughtered by a special procedure called shechitah, in which the animal's throat is quickly, precisely and painlessly cut with a sharp, perfectly smooth knife (called a chalaf) by a shochet -- a highly trained, Torah-observant and G-d-fearing individual.

An animal that dies or is killed by any other means is not kosher. It is also strictly forbidden to eat flesh removed from the animal while it is alive (this prohibition is actually one of the Seven Universal Noachide Laws and is the only kosher law that applies to non-Jews as well as to Jews).

Fish do not require shechitah.