Mark Baker has a nice, short write-up of how document style Web services differ from RPC-style and how that's related to state transfer (the last part of Representational State Transfer, or REST). An interesting point that Mark makes is that multi-method protocols (like HTTP) affect the semantics of the message. POSTing a message has different meaning that PUTting the same message. While this seems obvious after you say it, I'm not sure it's a point explicitly recognized by some people.