The stuff produced by your immune system in its attempt to get rid of (usually) a bacterial infection by the process of suppuration. Pus is mostly made up of phagocytes and bacterial cells - it looks white or yellowish and is icky.
Drainage of a collection of pus is usually a good idea.
Pus is also the word from which the word "pussy" has another meaning that has nothing to do with an English slang for a part of the female genitalia.