Mr. Clifford's explanation of aggregate demand, aggregate supply, and long run aggregate supply. Keep in mind that the "long run" is not a specific amount of time. The long run refers to enough time for resource prices (like wages) to adjust when there is a change in price level.
Here is another part of the puzzle: http://www.youtube.com/watch?v=7ObnvX64QWc
Here is a MINDMAP of Keynes' theory: http://www.buzan.co.za/gallery/KeynesModel-l-2.pdf