You can read about it from the source:
It takes patience to get used to the anal writing style, but these w3 documents lay it all out.
Reading w3 RFC's isn't everyone's idea of beach reading.
Maybe you'd prefer walking through a more summarized tutorial: [w3schools.com
A WDSL describes a web service. It tells you where to send the request, and what ingredients to include in the request.
Most XML services that use WDSL's are also expected to be accessed via SOAP.
I have to concede, WSDL's are pedantic. It's old-school, complex, invented by geeks who loved building things that were esoteric and annoying. I swear many of the archaic early technologies surrounding XML (like SOAP) seem like they were invented to frustrate programmers with a condescending LEET attitude.
That's an attitude I despise, BTW.
I've been working with XML for about 12 years. I've built hundreds of apps and mashups and web services, including dozens of API's and such, many (most) of them consuming XML in one way or another. Wanna know how many XML services I've used that offered their schema as a WDSL? One. It was a SOAP interface to a shipping calculator from UPS, back in 2001.
And I remember being frustrated trying to wrap my head around WSDL then.
One of the better explanations of WDSL is here:
You might enjoy its irreverent tone