Oil is a resource that is getting harder and more expensive to locate, extract and process. Much of the "light, sweet crude" has been used up.
An American geologist, M. King Hubbert, predicted in
1956 that United States oil production would peak between 1965 and 1970 - and he was correct. The same technique has been used to predict that world oil production would peak somewhere around now. The peak doesn't mean that the world is out of oil - just that it becomes harder and harder (and more expensive) to extract the oil that's left in the ground.
Peak oil isn't a new idea. There's even a society dedicated to researching peak oil (see ASPO: a network of scientists and others, having an interest in determining the date and impact of the peak and decline of the world's production of oil and gas, due to resource constraints.
http://www.peakoil.net/).
We won't know if we've reached the peak until a few years after we've passed it. It's best to start preparing now.
Somehow the rest of the world survives on much higher oil prices.