[wezfurlong]

At a high level, they make it easier to write server (or client) software that deals with multiple concurrent connections (such as HTTP server software, or just about any network facing service these days). They do this by abstracting some of the details away so that you can focus on writing your application code; instead you declare callbacks that get invoked when you have data available.

Traditional eventing frameworks focused on non-blocking I/O on a single thread on the basis that you don't need so many resources to scale up to a large number of clients when compared to a simple one-thread-per-client model.

There's lot of good material discussing this at http://www.kegel.com/c10k.html

libphenom is a bit more than just an eventing framework though; we have a number of APIs that help with putting together the whole application. And we blur the lines a bit: we also have thread pooling and dispatch support for cases where your can't build 100% of your application in a non-blocking fashion.