A display usually consists of a terminal screen, keyboard and mouse. Frequently the display and the workstation are the same. This is not always the case as a display can have multiple screens. A display is controlled by a special program known as an X server. There is only one X server per display.
Clients are user programs such as text editors, clocks and calculators.
A client receives all of its input and output through requests to the X server. The server handles all of the input and output to the display. It creates windows, produces text and graphics on the screen, etc. Clients cannot access the display directly, they must go through the server.