An oil well gets its name from traditional water wells. It is a hole that is drilled into the ground to reach a reservoir of liquid - in this case oil.
However, an oil well can be a little more complicated than a water well - especially oil wells under the sea bed.
First of all, the drill thread has to be lowered to the sea bed before it can start drilling through the cap rock.
Secondly, the well often spans out horizontally once has penetrated the reservoir. This is because the oil is held in porous rock - it is not like an underground stream. Often the oil has to be pumped out of the rock. It is only oil close to the end of well that is retreived.