Windows 7 comes in six separate variants:
Starter (OEM only)
User Interface features are similar in Home Premium, Professional and Ultimate. However, Home Basic lacks the Aero UI with Aero Peek, Aero Shake, and Flip 3D all missing. Live Previews in Windows Explorer is also not available. The Starter Edition offers a very basic UI and apart from what's missing in Home Basic, it lacks Live Taskbar previews also.
Home Premium, Professional and Ultimate support two physical processors, whereas Home Basic and Starter are limited to single processor support. Physical processors is not equal to multi-core and all variants offer multicore support. Also, Home Basic and Starter only come in 32-bit versions resulting in a limitation (below 4 GB) to the maximum RAM supported. 64 bit Home Premium can however support 16 GB and Professional and Ultimate 64 bit offers support for up to a whopping 192 GB of RAM.
Ultimate variant has BitLocker, and BitLocker To Go encryption technology which is missing from all others. Also, Professional and Ultimate offer backing up to a network and encryption of the File system which is not there in all Home Variants and the Starter Edition.
Home Premium, Professional and Ultimate come with its own Snipping Tool, Sticky notes and Windows Journal. These utilities are missing from Home Basic and the Starter Edition.
Home Basic and Starter variants do not offer Windows Media Center, DVD playback, MPEG-2 decoding and Dolby Digital compatibility like Home Premium, Professional and Ultimate. Home Basic and Starter do not support TV tuners while the other variants can support up to 4 tuners each of analogue and digital TV.
Home Basic and Starter can only join a HomeGroup and not start HomeGroup sharing whereas other variants can start their own HomeGroup. Starter Edition does not even offer Internet Connection Sharing and bridging of networks, a feature available in all other variants. Hosting a Remote Desktop connection is only supported on Professional and Ultimate, however, all Home variants can connect via Remote Desktop. Also, Home variants lack the utility to make files available offline.
Home Basic and Starter do not offer Tablet-PC functionality and multi-touch support, something that comes by default in Home Premium, Professional and Ultimate. Also, Windows Mobility Center in Home variants (Home Basic and Home Presentation) lacks presentation mode present in Professional and Ultimate.
This is one area where Ultimate scores big above all other variants. Several features such as Applocker, Booting from VHD, DirectAccess, Federated Search, Multilingual User Interface Language Packs, BranchCache and a subsystem for UNIX-based applications are only available for Ultimate and if any of these are essential to you, Windows Ultimate is your only choice. Also, Windows XP mode, Location-Aware Printing and the ability to join a domain or company networks are only available in Professional and Ultimate. Home variants miss out on almost all enterprise features, but nothing surprising about that.