What is a barrel?

I saw the oil price zooming past US$120. It's really amazing how the oil price keeps going up. One strategist for Goldman Sachs predicted it will even reach US$200 per barrel. The intersting bit is that how do we measure oil barrels? I am recycling this entry which I wrote two years ago.One of the interesting thing you find about Bruneians is that a lot of us don't know much about oil which is