To define and describe a web-service API, many developers would use WSDL. Although WSDL is meant to be extensible to any protocol and message format, most people use it for HTTP GET/POST and SOAP, when writing to WS-* standards. On the other hand, developers writing a REST API using XML over HTTP typically don't use WSDL, or any other standardized definition/description of the API. There will usually be some kind of human-readable documentation, but that's as far as it goes.
Not everyone is convinced that we need to describe and define REST-ful APIs, but there are those who believe it's useful. Tim Bray suggests that it's what we need to allow users to consume an XML/HTTP API in a few lines of code. It's certainly true that machine-readable descriptions of web APIs can allow the generation of a language-specific library.
