DirectSound is included with the DirectX application programming interfaces in the Windows operating system (and along with DirectSound3D, is labeled as DirectX Audio). DirectSound provides an interface between applications and sound card hardware. It is used with Windows XP and earlier versions of the popular operating system, but is not used directly with Windows Vista due to a completely re-written audio-stack based on the Universal Audio Architecture, blocking a direct path from DirectSound to audio drivers. DirectSound carried out many tasks in use as a vital service for Windows Audio, such as providing capabilities to recording sound from a input device and mixing it, or using hardware to accelerate tasks and allowing several applications to access the sound card simultaneously.
It was originally developed for use with gaming, but it has been used with professional audio applications due to the years of development that made it a mature API and its extra useful features, such as being able to play multichannel sounds in high res.
Other DirectX related glossary:
DirectX Media Objects