Tim Bray talks about GET and POST and REST vs. XmlRpc or SOAP.
Ways of Talking to a Web Server If you're a piece of software that wants to talk to a Web Server (a browser, for example, but there are lots of others), you generally use the HTTP protocol, which supports all kinds of transactions, but the two most often used are called GET and POST. GET is simple; you send a request encoded up in a URI to the server, and you get a package of info back from the server. This is what happens when you click on a hyperlink; nothing could be simpler, and a huge majority of all Web transactions are GETs. There are some rules that go with GET; it's not OK to use it ot make a purchase or trigger an explosive device or anything that changes the state of the world. It's for fetching information. When you POST, you contact the server and send it a package of information, not just a URI, and get back a package of information. You should use POST if you want to do anything more than just retrieve information, for example buy or blow up something. So most times when you fill out a form on a Web site and press Submit, it's using a POST. All this is straightforward and causes little angst as long as the Web is just browsers and pages. But a lot of people want to use the Web for more; to syndicate weblogs, run search engines, even to do large-scale B2B.
How do you fit this into the Web's GET/POST world? Well first off, you typically send machine-oriented XML rather than human-oriented HTML from the server. And if you ask for it with a POST, the package of information you send is usually XML too.
Source: http://www.tbray.org/ongoing/When/200x/2003/05/12/SoapAgain