1st one is the full version, which means you can install it on a PC with no previous version of windows currently installed. It is also retail, which means it won't be tied to the first motherboard you install it to. You can install it to a different PC if you wish, as long as you un-install it from the other PC first.
2nd one is just the upgrade version, you need to have a previous version of Windows installed on your PC and just upgrade that to Windows 7.
3rd one is also the full version, but it is OEM. It basically means it will be stuck to the first PC you install it on. It's not mostly the case though, a lot of people are able to install OEM copies on a different PC so it's like a retail copy but cheaper