Sic is a Latin word meaning "thus", "so", "as such", or "just as that". It is used to mark a mistake (or simply unusual) spelling, statement or idea. Its purpose is to make clear that the oddness comes from the original source, and you are merely repeating it because you feel it's relevant. "I know it's not spelled that way, but they did it!"
Mostly it exists so that people don't have to wonder if you accidentally misspelled something.
- For further information, see: Wikipedia:sic