As with all commodities that are readily supplied by several countries, crude oil can tend to either be in over supply or short supply.
When there is a short supply, prices bid up to the highest that the market sustains. When supply is ample, prices sink to the lowest that producers will accept. There is no happy middle ground. That is why commodity prices (not only oil, but cocoa, metals, etc.) are "volatile" and why it is so easy to lose your shirt in commodities trading.
So along with traditional supply and demand economic issues, there is also an incredible incentive to manipulate the market for a commodity.
For example, every time OPEC (Organisation of the Petroleum Exporting Countries) gets together and discusses cutting production, oil prices go up. If Iran pursues an atomic bomb and openly announces every milestone in their programmme, oil prices go up.
The knock on effect is that products made with oil, copper or silver get more expensive, manufacturers pass on the higher costs, prices among goods and services re-adjust, and life goes on.