Meet Jack. Jack is where things start to get weird. Most operating systems, err, Windows and OS X, provide an invisible interface to your audio hardware. In these OSes audio applications output to a software mixer, which mixes the signals and streams it to the sound card. This approach is similar to how ESD works. Similarly, Jack is an audio daemon that sits between audio apps and ALSA. Where Jack differs is that, in the proud tradition of Linux, it is infinitely configurable. This allows lower latency and the ability to pipe any output to any input, like a rousing game of Twister, but with data. Ardour requires it. Audacity and SND can use it. Any ALSA program can use it via an ALSA plugin.

ArsTechnica