In Judaic tradition, a descendant of Aaron who was chosen by g-d to perform various rites, including animal sacrifice, at The Temple. Plural is kohanim.
While being a kohanim isn't an important a role as being a rabbi or a rebbe, it is considered important enough that the kohanim lineage has been tracked down through uncountable generations. For the most part, the only real benefit to being a kohein today is making the first aliyah on the Sabbath.
The word kohein is the source for the surname Cohen, although there are certainly many people named Mr. Cohen who are not kohanim, and many kohanim who are not named Cohen. You will occasionally (rarely) seen cohen used as a synonym for kohein.
Kohein is pronounced KOH-hayn; Kohanim is pronounced koh-HAHN-eem.