From The Urban Dictionary:
2. Slang for a person (most generally a male)with the most abhorrent and annoying personal characteristics. Douchebags may possibly play rap loud music with their car windows down, wear oversized and sagging or ripped jeans, wear collars up, and wears sideways hats. Douchebags typically have low self-esteem (although appear to have an inflated ego), low IQ (although think they know everything), and small penises (though are constantly talking about how large theirs are). Douchbags attempt compensate for their shortcomings by their choice of vehicle. They likely are to drive huge gas guzzelers while preaching to the rest of the world to drive Al Gore approved Hybrid Douchemobiles. The demographic profile of the typical douchbag is an upper-middle class member of the Mainstream media.
*See also Kanye West