How do I write a service layer specification?

Question

I'm currently building a frontend prototype for a web application, and soon the backend programmers are going to start building the service layer. They've asked me to write a simple specification of the data formats I want from the various services along with possible parameters etc.

Are there any standard way of writing or structuring such specs? Can anyone give an example?

PS: All the service requests will be done in Javascript asynchronously.