SDL (Simple DirectMedia Layer) is a general API that provides low level access to audio, keyboard, mouse, joystick, 3D hardware via OpenGL, and 2D framebuffer across multiple platforms.
The current version supports Linux, Windows, BeOS, MacOS, MacOS X, FreeBSD, OpenBSD, BSD/OS, Solaris, IRIX, and QNX. The code contains support for Windows CE, AmigaOS, Dreamcast, Atari, NetBSD, AIX, OSF/Tru64, RISC OS, and SymbianOS, but these are not officially supported.
It is one of the most common game APIs. The other one is Microsoft's DirectX.
It was originally intended to port Windows games to Linux. It has served to port important games to that platform like Quake, made by Loki Games.