Old source code control solution from Microsoft. Superseded by Microsoft Team Foundation Server.
Microsoft Visual Source Safe was the Microsoft solution to source control, up to 2005. Its main asset is a good integration with Visual Studio. As of 2011, you should not consider this as your source control solution, except for supporting a legacy setup.
Largely superseded by Microsoft Team Foundation Server.
Some references about Sourcesafe:
MSDN Reference : http://msdn.microsoft.com/en-us/library/ms181038%28v=vs.80%29.aspx Microsoft Extended Support ends on 7/11/2017 : http://support.microsoft.com/lifecycle/search/default.aspx?alpha=Visual+SourceSafe
Caveat :