David

I am assuming that you are talking about the SOAP web services.

A web service session is an additional logon. So a user who is logged on through the main application interface can also have their credentials used to create a single extra webservice session. Every enabled user within CRM (this excludes resources and disabled users) has the ability to have this additional session. These sessions each have their own sessionid and the logons are recorded in the Activity table within CRM.

You can also see live sessions including web service sessions using the sysadmin path.

myserver/.../sysadmin