Windows Server 2008 = Windows Vista with Microsoft's Server Stack
Windows Server 2008 R2 = Windows 7 with Microsoft's Server Stack
There are multiple editions of Windows Server such as Standard Edition, Web Edition, etc. which bundle the software/management tools as required.
You can use either of the Server Operating Systems as a desktop system, however lot of configuration has to be done as in configuring the Desktop experience, and also note some software particularly the AVs will refuse to install in the Server edition of the OS.
Games run fine in Windows Server, boot up time is higher though but that can be fixed.