DirectX an interface b/w PC hardware and Windows

At the most basic level, DirectX is an interface between the hardware in your PC and Windows itself, part of the Windows API or Application Programming Interface. When a game developer wants to play a sound file, it’s simply a case of using the correct library function. When the game runs, this calls the DirectX API, which in turn plays the sound file. The developer doesn’t need to know what type of sound card he’s dealing with, what it’s capable of, or how to talk to it. Microsoft has provided DirectX, and the sound card manufacturer has provided a DirectX-capable driver. He asks for the sound to be played, and it is – whichever machine it runs on.

From my point of view as gamer, DirectX also makes things incredibly easy – at least in theory. We install a new sound card in place of your old one, and it comes with a DirectX driver. Next time we play your favourite game we can still hear sounds and music, and we haven’t had to make any complex configuration changes.

Originally, DirectX began life as a simple toolkit:
early hardware was limited and only the most basic graphical functions were required. As hardware and software has evolved in complexity, so has DirectX. It’s now much more than a graphical toolkit, and the term has come to encompass a massive selection of routines which deal with all sorts of hardware communication. For example, the DirectInput routines can deal with all sorts of input devices, from simple two-button mice to complex flight joysticks. Other parts include DirectSound for audio devices and DirectPlay provides a toolkit for online or multiplayer gaming such as networking.