The term Oil Country Tubular Goods(OCTG) describes tubes that are used in oil and gas production. This can mainly include drill pipe, casing or tubing pipes.
Drill pipe refers to the heavy tubes, usually seamless, used in the rotation of the drill bit and also the drilling fluid, to which they can be joined together using tool joints. The pipe is subject to axial tension (due to it’s own dead weight while in use), high internal pressure and extremely high torque.
Casing Pipe is the pipe used to line the borehole, which is the actual hole dug into the ground to get to the oil. Like the drill pipe, it too is subject to axial tension.
Tubing Pipe is the pipe that goes inside the casing pipe. This is the pipe that the oil itself will eventually make its way out of.