A Security Token Service (usually abbreviated to STS) is used to issue tokens. It is used for situations where a service requires a particular identity token, for example a SAML token. Check out
a video of an STS in action here. A client requests the token from the STS, using a SOAP request according to the WS-Trust RequestSecurityToken (RST) specification:

However, Vikas Jain
has suggested a more streamlined way for an STS to operate. Rather than using heavyweight SOAP messages to invoke the STS, why not use REST instead. This becomes a "RESTful STS". Vikas
has suggested a token exchange for clients of a REST STS.
Let's explore how a RESTful STS would be configured in Vordel's
XML Gateway. Here is a policy which will issue a SAML token or a more streamlined custom token when invoked with a RESTful query:

The policy is checking which type of token the REST caller is requesting: SAML or a custom token. The custom token is a lightweight non-XML token. Here we request a custom token from the STS using a request like this:
http://xmlgw:8080/RequestSecurityToken?TokenType=custom&RequestType=Issue
and we see the token returned:

Here we are requesting a SAML token using a request like this:
http://localhost:8080/RequestSecurityToken?TokenType=saml&RequestType=Issue
And we see the SAML token returned:

Calling a RESTful STS is, by design, must simpler than calling a full WS-Trust/SOAP based STS. Calling the RESTful STS from an XML Gateway is straightforward. In order to call the RESTful STS, we can make use of the fact that, within the Vordel
XML Gateway, you can call out to another service "off the the side" within a policy. Here we call out to a RESTful STS in order to request a security token, and then we inject it into a HTTP Header:

Testing this in SOAPbox, we see the security token inserted by the Vordel XML Gateway into the Authorization header. This header can then be consumed by the downstream application server, or an agent inside the application server.

In summary, a RESTful STS is certainly a promising way to make STS functionality more generally used, since it doesn't require a heavyweight SOAP message to be created.