Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Simply stated, it is a Solid State Drive. It doesn't spin like a traditional hard drive. The read and write times are generally much better than a standard hard drive. They make everyday tasks much snappier and draw less power(for the most part). The cost per GB is higher, but the performance benefit is worth it IMO.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.