ChatWho watches your channels to give you an idea of how many acquaintances are online with you.
For your guild and each custom channel that you're on, it displays and keeps updated a count of how many people are online. And mousing over the icons will present a list of some of the people that are on that channel. And when people join channels, those channels are briefly highlighted.
v1.1 implements data feeds for LibDataBroker, allowing flexibility and integration with other UI Addons.