PuTTY if just a client for telnet and SSH.
If you want to know how it work, try reading the RFCs.
Go to this page, and type "Telnet" in, click the "RFC" radio button on the right, and get any RFC dealing with telnet:
http://www.rfc-editor.org/rfcsearch.html
This page has a little more guidance, but it's links are broken - write down the numbers and read them at rfc-editor.org:
http://www.omnifarious.org/~hopper/t...elnet-rfc.html
Hope this helps.