Frankly, what I would suggest that you do is to fire up, say, the Firebug debugger in Firefox and look at the HTTP transactions that take place between the client and the server.   You will see first-hand how the information is encoded and transferred.   If you then examine the documentation for, say, JQuery, alongside the corresponding documentation for CGI, you will then see how each side is equipped to deal with the transfer.

The key point here is that there are two computer programs in play, written in two entirely different programming languages running on two computers, and they are exchanging data with one another using the agreed-upon protocol that is called "http."