Ident

"The Ident Protocol, specified in RFC 1413, is an Internet protocol that helps identify the user of a particular TCP connection. One popular daemon program for providing the ident service is identd." Wikipedia

What this means for 99% of you, is that if you have a ~beside your username in your hostmask on irc, you do not have an ident server on the machine you are connecting from, otherwise, you do.