The usual components of a drill string

The usual components of the drill string are: drill bit, drill collar, drill pipe, stabilizer, special joint and kelly.
The basic functions of the drill string are:
(1) Trigger the drill bit;
(2) Apply weight on bit;
(3) Transmission of power;
(4) Transport drilling fluid;
(5) Carry out special operations: squeeze cement, deal with underground accidents, etc.