IDENT is a simple protocol by which a server can request the username (or a cookie uniquely identifying a local user) associated with a client machine's TCP connection. It uses Port 113 and is sometimes also called AUTH.
The IDENT protocol was intended to report abuse in times when users generally only had unprivileged accounts on a centrally administrated host. It has become useless now that everyone is their own PC's administrator. It also doesn't transverse NAT very well due to port rewriting which makes it hard to tell which host the IDENT request is addressed to. Therefore it is mostly deprecated now, even though it is still in use on many IRC networks.