Basically with retail you can install it as many times as you like, but only on one machine at a time.
With OEM, it's restricted to the computer you first install it on. When you get a new PC, you need a new OEM license. You can get around it by lying to Microsoft and getting it re-activated on your new PC, but you'll be breaking the license agreement. Peoples' opinions on this vary.
Also, if you buy retail, you get a massage off an Asian super-model during the installation. With OEM you get a massage off a gay wrestler.