Comet, in general, is the technique of enabling server-side push instead of having client-side pull for refreshing information in the user's interface. The latter involves regular pinging of the server from the client to check for any changes that may have occurred on the server and send the corresponding information to update the user's UI accordingly. In applying Comet, one can notify the server-side changes to the client without the need of the constant pinging. Typical scenarios where Comet appears to have a relevant niche involve browser based chat applications, online collaboration and editing systems (such as GoogleDocs) and browser based web OSs.
The image below shows the chat application in operation in two different browsers on the same machine. The example was written in Visual Web Developer 2008 using the Microsoft Dot Net Framework 3.5. The code works in FF2 and Opera 9.5 so far. Not too many optimizations have been performed as the goal of this was to just demonstrate a way in which Comet could work without having to switch to any "dedicated Comet" servers. The same principles could be applied in Java too where the two connections could be established with two separate servlets to achieve the same results. In case anyone would like to view the source code for this, email me at email@example.com.