WADL

WADL stands for Web Application Description Language and is the REST equivalent to WSDL. An important difference is that the WADL is not required.

A REST based web service (REST Service) may be defined by a WADL descriptor file. It may look like this:

<application XMLns="http://wadl.dev.java.net/2009/02">
     <resources base="http://example.com/api">
         <resource path="books">
             <method name="GET"/>
             <resource path="{bookId}">
             <param required="true" style="template" name="bookId"/>
             <method name="GET"/>
         </resource>
         </resource>
     </resources>
</application>

A WADL definition is similar to WSDL in the way that it defines what you can do. It defines the supported HTTP verbs. In this example we can see that we are able to get information about books using the HTTP method GET.

We can also notice that there is a required resource that must be part of the request.

More information is available in the W3C standard suggestion