DAWs or Digital Audio Workstation is the musician's best friend, next to his or her MIDI keyboard that is. It's an electronic system that is designed for the purpose of recording, editing and playing back digital audio.
DAWs typically consist of a mixing console, control surface, audio converter, and a data storage all in one nifty package. A computer based DAW will need a computer, a sound card, a digital audio editor and an input device - your keyboard controller for example.
For the amateur musician and avid mac & apple user then nothing beats Garageband '11 - the latest software in Apple's amateur musician software line-up. You can get one when buying a new Mac, as part of the iLife'11 suite or as a standalone from the Mac Apple Store.