I'm making a site which should (a) take information from the user in the form of files and forms, (b) take that data to the server to be run on a C application, and (c) take the result back and show it to the user. I was told to look for AJAX for the communication with the server. BTW, I'm using rails.

I'm trying to understand how AJAX works. From what I understand so far, with rails is pretty easy to make the call.

What I can't figure out is, what waits for that call? what process the call?

If I understand correctly, with rails I could make a function in ruby and make it so it's called through AJAX, which means -or so I understand- that it gets executed on the server. If I were using PHP, would I need to make an http server to wait for the AJAX calls?

I just don't find information about what waits for the call, and that information is processed. Any links, comments or books are welcome!