Online chat is a generic term for what are now mostly known as instant messaging applications—computer programs that enable two-way typing to connect users to each other.

However, there are net dating systems and other online services that do not use these applications to provide chat facilities, e.g., Delphi, often relying on HTTP-based protocols instead. Sometimes these are called a chat room or chat extending the physical metaphors of 'sites' and 'magazines' which many web services exploit.

Today there are many chat rooms, some incorporating instant messaging features without having to install additional chat software. These chatrooms often have friend finders built in.

Some of these systems also provide telephone voice mail access. These are usually known generically as just chat systems.

A chat log is a record of a chat. Sometimes this is put on the web.

The earliest online chat system widely used was probably the Unix talk facility. The earliest one in general use was likely the French Minitel. During the 1980s, online chat was made available on CompuServe and DDial servers, and later on GEnie. In 1988, Internet Relay Chat (IRC) protocol, an instant messaging tool, was developed and became the de-facto standard for online chat.