The Jack Audio Connection Kit is a professional sound server for low latency audio and MIDI connections. When running JACK you can interconnect audio and MIDI ports between any JACK aware programs. It is a very flexible system that can allow for very modular set ups, but is also used to manage connections within various pieces of software, eg. Ardour and Qtractor. It can also allow you to sync up various programs, for example, if you want to use the audio sequencer in one program and the MIDI sequencer in another (see session management on how to manage modular set ups). More info can be found on the JACK website.
Qjackctl is a popular GUI frontend for managing JACK