Of or relating to someone who really, really likes to have lots of stuff.
Of course, nearly all of us want to have nifty stuff. We'd all like to have a nice new car, a nice new computer, nice clothing, and a gold-plated pogo stick, but the material girls and boys carry that desire to an unhealthy extreme.
Materialistic people want to possess expensive crap more than anything. They derive happiness from having things that other people don't have. They believe that "he who dies with the most toys, wins." They don't value people or ideas -- just material.
Are they right? Almost certainly not... but they can usually buy and sell the rest of us like cheap candy...