Client OM Architecture / Client Object model in sharepoint 2010

As can be seen in the above diagram all versions of the Client OM go through the WCF Web Service named Client.svc. This Web Service is located, along with all other SharePoint Web Services, in the folder [SharePoint Root]\ISAPI. The Client OM is responsible for packaging any requests into XML and calling the Web Server, however, when this call takes place is controlled by the developer as you will see. The response from the Client.svc Web Service is sent as JSON and the Client OM is responsible for transforming this into appropriate objects for use in the environment the call was made from.

                        /Client.svc/ProcessQuery                                      

The Client.svc Web Service has one method, ProcessQuery which takes a Stream object, an XML formatted stream as indicated above, and returns a Stream, JSON formatted. 

The implementation for this method can be found in the assembly Microsoft.SharePoint.Client.ServerRuntime.dll and the private class ClientRquestServiceImpl. Delving into this method, the first thing you find is an authentication check.