I suppose you mean the WSDL service description, which does not exist in WebAPI as such. Now I see you also added the tag REST to the question and I would like to stress that the WebAPI does not enforce REST practices.
REST is in the middle of some discussion lately, but I would think everyone agrees it does not define any wsdl-like service.
It does however require you to make your documents self-describing and encourages publishing possible state transfers and links to related documents
more info here: http://en.wikipedia.org/wiki/HATEOAS
REST is a very different beast than SOAP because it actually does not want you to send commands to and from the server other than the basic ones needed to get/update/.. resources, but a full description of this is beyond the scope of this post :-)
That being said, the webAPI technology in itself does not have any reservations in that regard and it would be perfectly possible to put full-blown services and a related wsdl like structure in place.