http://en.wikipedia.org/wiki/Alignment_%28Dungeons_%26_Dragons%29

This should explain a bit better than I could. 

They're a bit smarter than humans, so they learn relatively quickly. In terms of physical strength, outside of water they're at or a little bit below humans. In terms of magic, they're fairly talented and are comparable to elves. Their physical abilities increase a great deal when submerged, however.