For the sake of accuracy: originally ‘chat’, ‘online-chat’, referred to sites in the WWW where you go to meet people in the internet in so called ‘chat rooms’. You visit such a place, log in and ‘informally converse’, chat that is, with those also online. You communicate by text messages that can be read by all those logged in. There are many places for many interests and people.

Nowadays ‘chat’ is also used for ‘Instant Messaging (IM)’. The difference to the ‘original’ is when you’re visiting a ‘chat room’; you’re talking to many people at the same time, whereas when using an Instant Messenger you directly choose interlocutor(s).