Yep that's basically it.
OEM ties the license to the "device", which microsoft take to mean the motherboard. So you can usually get away with adding a new video card and a new processor etc without needing to phone them, but if you change the motherboard it's quite likely you'll have to ring them up. If it's because of a hardware failure they're usually pretty good about it.
The retail license allows you to transfer to another device (so long as you remove the installation from the original device first), and also gives you 90 days of free support incidents etc.