Yoni,
With a few simple words you've really asked a mouthful. This is a little tricky to answer because there's a lot going on between the client and the server(s). Renke is right in that there is ODBC and SOAP communications going on between client and server.
I guess I have to answer your question with a question: are you asking about network protocols which are generally considered to be OSI layers 3, 4 & 5 (layer 1 is the physical cable/hardware interconnects) or application protocols - generally considered to be layer 7?
If you're talking about layer 3, 4 and 5 then the answer is probably TCP/IP and UDP. Over those protocols travel many types of (standard) communications, which is where ODBC, SOAP, HTTP, HTTPS, etc (the layer 7 application stuff) comes into play.
There are tools that can be used to actually watch the traffic of a network session that can record each communication. These are typically refered to as sniffers or protocol analyzers. If you have network engineers available to you, you might think about asking them to "sniff" one of your project server sessions to see what's going on - assuming you've got project server running. ;>) Hope that helps.
Earl
Which protocols are used between the client and each Project 2000 and
Project2003?