"The wonderful thing about standards is that there are so many of them to choose from."An API, or Application Programming Interface, is a communication protocol between two sets of code. It's like a language; both parties agree that words have a particular meaning, so that they can talk to each other. Why? Because Software is Hard. Hardware makers do their best not to agree on how hardware should be made. Power requirements & connections have to be standardized, but how do you think they could actually make such a great XYZ5000 if they actually told you how they did it? Originally - especially when a new computer comes out - people write code to talk directly to the hardware. MSDOS games routinely wrote directly into the video memory for fastest results. When you start having multiple programs running simultaneously, they all can't do that, any more than everyone at a meeting can all talk simultaneously, so eventually APIs and driver programs would do the work, and programs would call them in order to play nice with everyone else. A driver implements a particular API that the operating system requires for interfacing with a certain kind of hardware. So hardware makers write 'software drivers' that allow your lowly computer to understand & communicate with the awesomeness that is their XYZ5000, at least on a basic level. So your computer can now handle the hardware, be it Microsoft Windows, macOS, or UNIX. However, Microsoft, Apple, or the various makers of the various *nix's are not stupid; they know there's malicious (or just poorly written) code out there. They are not just going to let any old program willy talk to drivers. Especially drivers relating to drawing stuff on the screen. The OS needs to know about this sort of thing, and it needs to manage access to that hardware. So the OS exposes an API that allows other programs to talk to the hardware. OpenGL and Direct3D are the two available APIs for dealing with 3D graphics. An interesting aspect of both APIs is that, because they directly access hardware, their features are very reliant on the underlying code. Previous graphics cards may work, as there's usually a built-in render path using older versions, but you usually won't see the new features. There are also programs which implement the API in software, although the featureset and performance is always well under what you can find in hardware. Frequently, how efficiently these pieces of Hardware work with APIs depend not only on the hardware, but on how nicely they play with APIs (part of software drivers). The amount of work put into this, in part, is determined by how much money they expect to make from them. In the future. So old hardware slowly migrates to the great big garbage dump in the sky, because there is no money to be made. There have also been cases of newer graphics cards omitting or otherwise breaking little-used features that some games during the Direct3D 7 and prior era used, either at the driver or hardware level, so don't expect perfect backwards compatibility. Sometimes you might just have to build an old computer to run a suitably old game well, or even at all. See the Trivia page for a list of popular/common API.
—The UNIX-HATERS Handbook