This graphic illustrates an example STS token exchange.
The scenario is as follows (is also described in the text below): User "joe" logs into his desktop and a Kerberos ticket is created. When the user opens the desktop application and performs an operation, this results in a backend Web service call and we want to propagate the identity of "joe" to the backend application. However the token we have is a Kerberos token on the client side and the backend Web Service only accepts a SAML token. You can use an STS to do a token conversion or token exchange.