All lumber is measured Pre-Planed. That means, when it comes out of the sawmill, it's 2x4 or 2x10" rough cut. When it goes through the planer, to make the boards smooth, this removes some wood, thus a 2x4 is acctually 1.5"x3.5" approx.
Heres a link to a chart
http://mistupid.com/homeimpr/lumber.htm