Remembering the screen

How can I, with OpenGL, do something like this? First a complex scene is drawn on the screen. Then, let OpenGL remember this screen. Then, draw something else on top of this screen (a window, for example). Then, bring back the scene it remembered (without the window on top of it). Is there a FAST way to do this, without having to redraw the scene polygon by polygon? I want somehow to be able to save the current screen and draw it back quickly at any time, without doing something like checking every pixel or polygon. Something like saving the current screen to a texture and then drawing that texture maybe, but I don't know how to do that and if it's the best way. I'm using OpenGL combined with SDL by the way, I don't know whether or not that's relevant.