An oil rig is a large machine that's used for drilling deep holes in the earth so that oil can be extracted. There are both on-shore and off-shore oil rigs.
An oil rig is almost always an enormous, complex structure that can drill miles into the ground to access oil. Some oil rigs are semi-permanent and include housing for the crews that work there. Smaller oil rigs are often mobile, and are usually moved once the drilling is finished so that another type of rig can begin to extract oil. A rig can be a truck, bus, well-drilling apparatus, or even a horse-drawn carriage.