Am I Buying the right OS?

I'm upgrading my OS from Windows XP to 7 Home Premium, and I think I've found who I'm buying it from. The only problem is that it's advertised under the name "Microsoft Windows 7 Home Premium SP1 32-bit - OEM". I know I'm running a 32-bit system, and I think that the fact that I'm running SP3 on my current OS doesn't matter, but I can't get my head around the "OEM". All I want is the OS on a disc I can pop in and install. Does OEM mean disc, or am I buying a download? Please Help!