One of the more mysterious ways in which you can legally acquire Windows 7 is through the so-called OEM Packaging. OEM stands for Original Equipment Manufacturer, which you can generally think of this term as meaning "PC maker" or, more precisely in this case, "system builder." And given this, you may assume then that Windows 7 OEM packaging is thus off limits to you as an individual.
This is mostly correct, as it turns out.
Previous to Windows 7, Microsoft actually did allow individuals to purchase OEM versions of Windows, but only if they intended to install it on a brand-new PC that they were building. As Ed Bott pointed out in a 2008 blog post, the previous end user license agreement (EULA) for OEM Windows versions specified the following:
OEM system builder software packs are intended for PC and server manufacturers or assemblers ONLY. They are not intended for distribution to end users. Unless the end user is actually assembling his/her own PC, in which case, that end user is considered a system builder as well.
This is pretty clear-cut. Sadly, this language does not appear in the licensing for OEM versions of Windows 7. Yes, individuals can still buy OEM versions of Windows 7. But they cannot then install that copy of Windows 7 on a PC they are building for themselves. Instead, they can only install it on a PC that they then sell--and support--to someone else. And they must do so using Microsoft's OEM pre-installation kit (OPK), a step that is clearly aimed at further preventing this type of software from being used by the hobbyist market.
Note: You can view Microsoft's current OEM system builder license here.
From a practical standpoint, OEM versions of Windows 7 don't make sense for individuals. So most readers of this site--unless you are a system builder, of course--can simply move on and proceed with the retail version of Windows 7 that makes the most sense for you.
Note: Curiously, anyone can, in fact, buy an OEM version of Windows 7. They're sold individually an in multi-packs at such online retailers as NewEgg.com.
From a technical standpoint, an OEM version of Windows 7 is roughly identical to a Full version of Windows 7. The discs are available in 32-bit or 64-bit versions, for all of the mainstream Windows 7 versions. However, these discs are only supposed to be used to clean install the operating system, and once that OS is installed and activated on a particular PC, it cannot be deactivated and moved to a new PC. The OEM is required to provide Windows 7 support to the user that buys the PC on which it is installed. The OEM must also stick the included Certificate of Authenticity (included with the OEM Windows 7 packaging) somewhere visible on the PC they are selling to an end user.
Here's what a typical OEM version of Windows 7 looks like.
Outer packaging, front.
Outer packaging, back, with EULA.
The Setup disc and COA.
Interior packaging, opened.
Close-up of the COA.
Update: Coincidentally, Ed Bott was also working on a related post about OEM versions of Windows 7 at the time I wrote this. That post, Is it OK to use OEM Windows on your own PC? Don't ask Microsoft, is available now and provides more information about this topic. It's a must-read.