Windows 7 / Windows Server 2008 R2 Service Pack 1 is now available on Windows Update, MSDN Subscriber Downloads, and TechNet. The announcement from the Windows team can be read on the Windows Team Blog, and one from the server team can be found here.
For Windows 7, Service Pack 1 is primarily a roll-up of fixes. Detailed documentation can be found on MS Downloads. SP1 includes some performance improvements for Direct2D/DirectWrite and the WARP10 software renderer (see KB 2028560), and provide OS-level support for the Intel Advanced Vector Extensions (AVX).
For Windows Server 2008 R2, Service Pack 1 includes the same fixes plus adds two new server features: Dynamic Memory and RemoteFX.
Update: Support for Windows 7 RTM ended on April 9, 2013. Mainstream support for Windows 7 with Service Pack 1 ended on January 13, 2015. Extended support for Windows 7 SP1 will continue until January 14, 2020. See the Windows lifecycle fact sheet. In short, if you are still using Windows 7, it needs to be Windows 7 Service Pack 1.
VS 2015: Note that Visual Studio 2015 can target Windows 7 Service Pack 1, but the VS 2015 C/C++ Runtime does not support Windows 7 RTM.