JACK is a low-latency sound server, allowing multiple applications to connect to one audio device, and to share audio between themselves. Many Professional-grade programs (such as Hydrogen and Ardour) on Ubuntu utilize JACK for maximum hardware compatibility and performance.
Have you ever wanted to take the audio output of one piece of software and send it to another? How about taking the output of that same program and send it to two others, then record the result in the first program? Or maybe you're a programmer who writes real-time audio and music applications and who is looking for a cross-platform API that enables not only device sharing but also inter-application audio routing, and is incredibly easy to learn and use? If so, JACK may be what you've been looking for.
JACK is system for handling real-time, low latency audio (and MIDI). It runs on GNU/Linux, Solaris, FreeBSD, OS X and Windows (and can be ported to other POSIX-conformant platforms). It can connect a number of different applications to an audio device, as well as allowing them to share audio between themselves. Its clients can run in their own processes (ie. as normal applications), or can they can run within the JACK server (ie. as a "plugin"). JACK also has support for distributing audio processing across a network, both fast & reliable LANs as well as slower, less reliable WANs.
JACK was designed from the ground up for professional audio work, and its design focuses on two key areas: synchronous execution of all clients, and low latency operation.