By definition, the drill string is the drill pipe with the tool joints attached. The drill stem consists of the drill string
and all other attached members, including kelly, subs, drill collars, heavy weight, drill pipe, stabilizers, shock
absorbers, reamers, and any other in-hole equipment used generally or part-time during drilling operations. More functions ditails about every sigle element has to be studied individualy. But the main function is transfering the power from top to the bit.
