There are two basic types of online "text" discussion, asynchronous or "on-going" and synchronous or "real-time" discussion (also known as chat). In asynchronous on-going discussion, there is a delay between when one participant sends their discussion text and when the other participant(s) receives that text. In synchronous or "real-time" discussion, the discussion text is sent immediately so that the recipient sees it soon after it was typed. For more information on this type of discussion, see: "On-going Discussion."
Instant messenging (IM) was one of the first types of synchronous or "real-time" discussion. AIM was one of the most popular IM services. Other services include MSN IM. More recently Google has introduced Google Chat that is integrated with your Google account and is accessible via Gmail.