... or "Summer Time" as it's called in some other countries:
http://encarta.msn.com/column/daylightsavingtime.asp
So, it serves three purposes.
Safety, Energy Conservataion, Crime Prevention. It was first suggested by Ben Franklin, and first put to use in England during WWI. In the US states don't have to follow it, but if they do they must do it the same way as everyone else.