WHAT IS TRIP HOP?

this is a lingering question in my mind. I have been listening to "trip hop" labeled bands for years. But I see so much diversity in the music that Im not sure what it is exactly. Is it a genre that just sets itself apart from other "pop groups" as a mind set? Or is it the elements in the music itself that makes a group labeled trip hop? I see so many random groups on this site. Im a little confused...is it the groove? Is it the mood? Is it the music? WHAT THE HELL IS IT? :?