Comet (also known as "Long Lived HTTP Connections" or "Ajax Push") lets server-side applications notify changes to its clients (Server-Initiated Message Delivery). Typical use-cases include chat, document-sharing application or much more ambitious architectures.
Bayeux is a JSON-based protocol specification of Comet and gCometd is the name of the implementation inside Grizzly 1.0.11+ (DOJO or Ajax only) and GlassFish v2 b35+ (JSP, JSF, servlet).
The Comet/Bayeux spec (0.1draft5) released by the Dojo foundation couple of months ago is now supported in GlassFish v2 build 52. Unfortunately, this specification breaks most comet clients. Jean-François "Grizzly" Arcand recommends that you update to the new DOJO library if that's what you're using (and you probably should be since writing a Bayeux client isn't trivial).
More generally on Comet, check out:
• Comet Basics
• A good presentation
• Writing a Comet web application using GlassFish
• This simple, yet very cool demo.