SDL = Simple Directmedia Layer.
It is a platform independant way to access a computers Graphics, Sound and other misc hardware.
A program witten using it can be compiled to run on almost any platform.
It is very fast as it always uses the fastest possible method to use the hardware.
It also supports OpenGL for 3D :-)
Physically SDL is a shared Library, sdl.library, and it is acess in the same way as the graphics.library, the advantage it has over the graphics.library is that it supports modern Hardware and is available for most operating systems.