Sort of like IRC channel, but not quite.
Usually, "chat room" refers to a system (often in Internet-accessible place, often in proprietary information service) in which people can chat in real time. (Usually "chat" refers to something that's not mentioned in polite circles.)
In Internet and World Wide Web, these are usually implemented with Java applets, Flash, or in some crazy situations with HTTP and "streaming" HTML (though HTTP/HTML is not designed for this use...). Usually they offer some quasi-nifty features like user icons or pictures, but they're usually technically inferior to IRC (mostly they need much more muscle from the computer and offer less *useful* features...)
Some people use the word "chat room" to mean a weblog or message board, but these uses of the word are incorrect (because they fail to meet the "real time" criterion).
(See also: Comparision of IRC and Web chats)