The Draw Works is the motor that reels in and reels out the drill line in order to raise and lower the Top Drive and Drill Pipe, however in its entirety the Draw Works is a package that includes motors, gears, cable spool, main brake, auxiliary brake and sometimes a cooling system.
Controlled by the Driller, the Draw Works will be turned to reel out the Drill Line. The Drill line travels up to the top of the Derrick, through the Crown Block and down and through the Travelling Block. This in turn lowers the Top Drive and any Drill Pipe that is attached. The rate of decent is normally controlled by the auxiliary break. This is can often be in the form of Eddy Current Motors and can be know as the Emalgo Brake.
To raise the Top Drive, the Driller will rotate the Draw Works to spool in. This will engage the electric or diesel motors powering the Draw Works. The Drill Line will travel through both blocks, giving the Draw Works a degree of mechanical advantage. When in position, the Driller will engage the main break, also known as the parking break.
Video courtesy of BenTEC