Groo the Wanderer is a parody of "brutal sword and sorcery heroes", which were very popular in the 70's (when he was created).
Groo the Wanderer was written and drawn by Sergio Aragonés (who contributed to MAD magazine). It has been published by multiple companies: Pacific Comics, Eclipse Comics (which only published one issue), Marvel Comics under the Epic imprint and Dark Horse Comics (which recently reprinted the first issue of Groo like Usagi Yojimbo for one dollar).
Groo is very stupid with a noticeably large nose. His stupidity and ignorance causes him to misunderstand what is going on around him (which usually leads him into trouble). He loves fighting so much, he will often jump head on into battle, without knowing what he is fighting for. Despite this, he is also fairly peaceful. He has extreme bad luck, to the point that stepping on a ship will sink it. He has become so widely known that Groo's approaching can cause massive chaos. The only thing he is good at is using his swords. He has been seen slaying entire armies with nothing
but his blades that seem to be katanas. Groo will often charge into battle while saying, "Now Groo does what Groo do best!" Ironically, he is peaceable and honest (when he isn't enjoying a good fight, that is).
In Other Media
The rights to a Groo movie have been sold. In 2008 Aragonés and Evanier they began a pre-production on the animated movie. There has been no other word about it since for over two years, so we can assume that there will not be a movie coming out soon.