Security Token Service
[Starting with the .NET Framework 4.5, Windows Identity Foundation (WIF) has been fully integrated into the .NET Framework. The version of WIF addressed by this topic, WIF 3.5, is deprecated and should only be used when developing against the .NET Framework 3.5 SP1 or the .NET Framework 4. For more information about WIF in the .NET Framework 4.5, also known as WIF 4.5, see the Windows Identity Foundation documentation in the .NET Framework 4.5 Development Guide.]
Security Token Service (STS)
A security token service (STS) is the service component that builds, signs, and issues security tokens according to the WS-Trust and WS-Federation protocols. There’s a lot of work that goes into implementing these protocols, but WIF does all of this work for you, making it feasible for someone who isn’t an expert in the protocols to get an STS up and running with very little effort. You can use a cloud STS such as a LiveID STS, a pre-built STS such as Active Directory® Federation Services (AD FS) 2.0, or, if you want to issue custom tokens or provide custom authentication or authorization, you can build your own custom STS using WIF. WIF makes it easy to build your own STS. For more information, see Building an STS.