Baking with rendering a complete scene

I think I believe what baking means in blender, but anyway, I will ask this: Is there a way that I can render a complete scene without it having to render everything over again, I have a city block with buildings and trees and I am also using the “Freestyle” and it taking lots of time to “Freestyle” the trees, the trees having leaves and trunks, each a lot of individual things. I have seen where some tutorials bake cloths by using the UV method, but I do not think that will work with me? I am using a laptop with only 8 gig of Memory, I tried rendering a city scene with trees, and the render was very very very slow…(without the trees, it renders fast)…Help!