Directx is an application programming interface (API), it’s a tool for developers to ease the process of creating code for applications, in this case, mostly for games.
The Directx API includes several components for 2D graphics, networking and input devices but most people here tend to use the term directx to refer to the 3D graphics component only, Direct3D, which is the most extensive part of the API.
A program in its most basic form is a series of instructions. Those instructions can be very simple or very complex but even the most complex instructions can be broken down into a series of basic instructions. What an API does is offer a series of complex instructions the developer can use without having to code every basic instruction behind it.
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