http://technet.microsoft.com/en-us/library/ee806864#section1
This article provides guidance to enable you to configure the Microsoft SharePoint Server 2010 security token service (STS). An STS is a specialized Web service that is designed to respond to requests for security tokens and provide identity management. The core functionality of every STS is the same, but the nature of the tasks that each STS performs depends on the role the STS plays in relation to the other STS Web services in your design.
An Identity Provider-STS (IP-STS) is a Web service that handles requests for trusted identity claims. An IP-STS uses a database called an identity store to store and manage identities and their associated attributes.
A relying party STS (RP-STS) is an STS that receives security tokens from a trusted federation partner IP-STS. In turn, the RP-STS issues new security tokens to be consumed by a local relying party application.
