Formerly by Emagic, currently by Apple. A DAW, MIDI sequencer, mastering environment, loop sequencer, softsynth collection, and more.

There are several versions; Logic Pro is the everything-plus-the-kitchen-sink version, Logic Express is more entry-level, and a kiddie version called Hit Kit. Much of Logic's innards also form the basis of GarageBand.

If you need help running any of those, ask fluffy.

For a sales brochure and current feature comparison, see http://apple.com/logic/