It typically includes the bit, a mud motor or a rotary-steerable tool, measurement-while-drilling tools, stabilizers, and drill collars. It puts weight on the bit, keeps the hole straight or steers it, and carries the surveying and logging tools.
Its stiffness and its stabilizers decide whether the assembly builds, drops, or holds angle. A long, slender assembly buckles if too much weight is stacked on it. The assembly is a measured-depth object: its components are tallied in feet of steel, and the surveys it takes are what convert that tally into true vertical depth.