Telnet was originally designed to be able to remotely login a server (host). Telnet is also one of the most basic protocols. But since you are refering to GET and POST i'm guessing you've been using telnet to connect to a webserver and issuing HTTP commands by hand. So if you want to know about GET and POST look for info on how HTTP works.

Edit: If you really want to know about the nitty gritty of HTTP, read the rfc here