What is SDL?
Simple DirectMedia Layer is a library written in C that provides low level access to audio, keyboard, mouse, joystick, 3D hardware via OpenGL, and 2D video framebuffer in a portable manner. In other words if you use SDL you'll have an easier time porting your game or application to all the platforms that SDL supports.
SDL is simple and only does the hard stuff for you (making your app portable). It does not have any networking, GUI or various image facilities for you. There are however add on libraries that will do that.