Anywhere that is selling any less that 10% or so of MS is usually a scam with a pirated key. You simply can't get them any cheaper from MS even if you buy in bulk.
Branded ones are usually tied to a particular company like Dell, HP, etc. You can't buy these, they are only sold directly to those companies.
Upgrade means you need a previous OS installed on your HD to install. Cheaper but a pain sometimes. If you need to reinstall, sometimes you have to install the full previous OS first, just so it can detect it, and then do the upgrade.
OEM/System builders. These are made for people making their own PC's. The problem with Windows 7 is it is tied to that particular hardware. Changing out a motherboard can result in the Windows becoming deactivated as you changed the hardware and that copy was only licensed to that particular set of hardware you installed it on.
Full is a full version, no upgrade needed, you can change hardware, do what you want with the OS. Most expensive.