Basically, demand and supply.
What OPEC is doing is to try to restrain the supply side (thru production quotas for each member country), so that prices can rise and they can get a better bargain. Though it doesn't work that well nowadays, since some oil producers aren't OPEC members and also because of political pressure...
Add: The high prices at the moment are because of increased demand, particularly fr China. Also 'cause the oil derivatives traders are betting on higher oil prices in the future, so...