The main description will fit in some kind of live chat support system. However, there are some big differences and advantages:

• You can open a chat window (inline or pop-up) to any online visitor.
• Run on multiple sites: one master centralized site and many clients (domains) to serve.
• Optimized for high traffic sites.
• Client sites just need add a small javascript, no php involved.
• Only open connections when some status change (admin logged, new chat, etc).
• Chat's script is really fast using filesystem for writing.
• Idem for reading, just show file contents via ajax.
• Show your online status.
• Log chat messages.