Chat channel (an integer) used by scripts for communication or avatars to talk to scripts. When a user is talking to a script the syntax is "/<channel> <text>", i.e. "/3 hello". Scripts can use both negative and positive values as chat channels, while avatars can only use positive numbers.
A "room" in IRC. A place on an IRC network where people can go to talk about a subject. A channel usually looks like "#<name>" such as "#secondlife".