[Frooxius]

Well you need to look into networking libraries. However definition "send requests to servers and receive their response" is extremely vague and includes a lot of various network technologies and protocols, that can differ a lot, both in the ways of communication and the type of communicated information itself.

You might try to look at this, but if you want more detailed information, you need to specify what exactly you want.
http://download.oracle.com/javase/tutori...rking.html