Richard Heinberg defines Peak Oil

The industrial revolution arose primarily because of fossil fuels. Fossil fuels contain immense embedded energy stored millions of years ago by plants and animals of earlier era's. The embedded energy represents manual labor which does not have to happen because the planet stored plant and animal carcasses converting the oils in the tissues into fuels. The wonders we've gotten as a society came because of the "free" energy in fossil fuel, but the quantity of this fuel is not a sustainable resource which will last our society for very much longer.

The basic model of peak oil (which refers to the peak of production) is that in oil field after oil field they've observed a bell curve of oil production. Average the production over the thousands of oil fields around the world, and it's a real bell curve. And, we are approximately at the top of the bell curve. Meaning that soon global oil production will start heading down the far side of the bell curve, meaning a decline in oil production capacity.

Richard Heinberg is a leading thinker in the movement of people bringing attention to Peak Oil. This is a very good presentation giving us a way of understanding the model.