Christian posted a link to a starting-point implementation of WS-MetadataExchange that's been put on top of ASMX web services through Soap Extensions. Read no further if you already know what that means.

Web services are all about the exchange of messages and we have WSDL (and its friend, XML Schema) to help us define what the message exchange patterns are like for a particular web service.

Beyond that, a web service may have a particular policy that it applies to messages that it sends or receives and that policy might state information such as whether the messages need to contain authentication information or whether they need to be digitally signed or encrypted and so on.

WS-Policy provides a generic framework for describing the policy requirements of a web service and other specifications (such as WS-SecurityPolicy) apply WS-Policy to a particular area that require policy assertions to be made.

When two services interact it's going to be handy for them to be aware of each other's policies in order that they can find some common ground between them that allows communication to flow. Without common ground on policy, the services are going to fail to communicate.

Today, there's not really a prescribed way of getting hold of a web service's policy.

There's WS-MetadataExchange which specifies one way of doing it and there's WS-PolicyAttachment which has a couple of ways of associating policy with a web service.