Definition - What does Drill String mean?
Drill String is a string or a column of a drill pipe present on a rig which is used to transmit the torque and drilling fluids to the drill bit. The term is applied to the loosely assembled collection of the drill collars, drill pipe, drill bits and downhole tools. It is hollow inside hence it allows drilling fluids to be pumped down and circulated back up to the annulus. It is a combination of the bottom hole assembly, drill pipe and other tools that are used to make drill bits turn at the bottom of the borehole.
Petropedia explains Drill String
A Drill String is supported by a top drive that allows it to advance to the downhole and rotates it on the surface for driving the bits. It is used to maximize the hanging length and strength by using the lighter drill pipe with 5 inches of the outside diameter on the bottom for reducing its weight and by using stronger drill pipe with 5.5 inches outside diameter on the top where the bending stress is higher. The Drill String is run using a dual elevator system that uses two heavy lift elevators for supporting a pipe-tool joint instead of the kind of tooth that dies in slips.