DirectX is a set of APIs that help software developers make video games for Windows PCs and Xbox game consoles. The DirectX APIs function as a middle layer between the game software and the computer's hardware, allowing developers to use a basic set of commands to control countless possible system configurations.
Windows device drivers for video cards, sound cards, and game controllers include DirectX support, which lets the API interact with and control the hardware. DirectX is not a single API — it is a family of APIs that each manage a single type of hardware. The Direct3D API, for example, controls a computer's GPU to render real-time 3D graphics. Direct2D renders 2D bitmap and vector graphics using a GPU's hardware graphics acceleration. The XAudio and XACT APIs control audio output.
Creating games using DirectX requires that the developer use of DirectX software development kit (SDK). Since both Windows computers and Xbox game consoles use DirectX, developers can use it to make games for both platforms at once. And, because DirectX is built-in to Windows and every device driver, the end user doesn't need to install anything except for their games.