A Digital Audio Workstation (DAW) is a piece of software designed for the rendering of a musical recording. It differs from a Notation program in that its primary purpose is to make audio recordings whereas a Notation program's primary purpose is to create musical notation in the form of scores and parts for performance by live musicians.
A DAW is essentially a full-blown music recording environment - set up in this case for midi keyboard recording, which can trigger and record almost any instrument sounds, as well as record any electronic instrument sounds (i.e. electric guitar/bass, syths, or other hardware-based electronic instrument. DAW's feature deep editing control of both recorded sound and midi data and are essential to produce and render high-quality and realistic-sounding recordings.
Some composers like to begin in a DAW, while some like to begin in a notation program. They often end up using both. There is some overlap in functionality but they can not do everything the other can.