This Park Tool page shows the difference.
http://www.parktool.com/repair/readhowto.asp?id=48 If it has 6 cogs at the back it will be a freewheel. If there are 8 or more it will be a freehub. If there are 7 it could be either. The bearing arrangement in a freehub makes it much stronger, so is the way to go, if you weigh 235 lb like me. If you weigh 150 lb it shouldnt matter to you, but is a good reason for getting the price down.