Looks like a fairly standard three-piece crank, which is pressed onto a square axle when you tighten that nut you removed. You get it off with a tool that first screws into the crank and then presses the crank off the axle. You can find one at any bike shop. My was from Park Tools.

Park Tools "How To" for crank removal: http://www.parktool.com/repair/readhowto.asp?id=103