A+: Versions of Win7 and requirements*

Windows 7 includes the Starter version (which limits how many apps can run at once) as well as Home Premium, Professional, Ultimate and Enterprise. Here's a partial reference to versions and features:

Features Starter Home Premium Professional Ultimate
 Win XP Mode no no Y Y
 Domain Join no no Y Y
 Backup to LAN no no Y Y
 Bitlocker Encryption no no no Y

* The formal minimum requirements for Windows 7 (which is never enough speed to keep any user happy)  are:

Component Requirement
Processor 1 GHz
Memory 1 GB (32-bit) or 2 GB (64-bit)
Disk space free 16GB (32-bit) or 20 GB (64-bit)
Video DirectX 9 w/ WDDM 1.0 or better driver
Optical drive DVD-ROM

The Windows Compatibility Center  allows you to check your Windows PC, as does the Microsoft Assessment and Planning Toolkit, or running {Win-R} | msinfo32.exe .