DirectX is an API, or "Application Programming Interface," that simplifies and standardizes the means by which developers program media applications for Windows. Developed by Microsoft, it is the foundation for most modern PC games and forms the foundation of XBox and XBox 360 game software. The DirectX SDK is the "Software Development Kit" that includes all of the libraries, "include" files and utilities that are necessary to write programs that will run under DirectX.

DirectX allows for substantial performance increases by utilizing hardware acceleration. This is the process of drawing 2D and 3D graphics on a specialized graphics processor that is typically found on a PCI Express, AGP or PCI card. This chip can also be located on the motherboard or embedded inside of a CPU. Because these processors are specially designed to perform actions such as color and texture fills, shader math and hardware transformation of triangles, they provide enormous performance gains compared to running the same code on the computer's main processing cores. DirectX automates and simplifies the process of utilizing these hardware features. It interacts with the hardware indirectly by communicating with the device drivers, such as AMD/ATi's Catalyst software.

Although the DirectX SDK is primarily intended for developers, it is also useful for running certain applications. Dynamic link libraries, or DLLs, that are included with the DirectX SDK enable some programs to run that would otherwise be unable to. However, it is strongly advised that the developers of such applications make an effort to include this content via static linking whenever possible so that users will not be expected to download a very large DirectX SDK installer file unnecessarily. Most programs that depend upon DirectX do not require SDK components in order to run.