Based on only a relay, this electromechanic memory cell may represent one data bit. The data record in the cell is performed through the 'set' button (1) or reset (0).
To reflect: imagine the size and device power consumption that used this type of memory. Believe me, with too few modifications, this was the basis the giant ancestors of your PC.
Note that the memory status is maintained by the configuration of the relay in the circuit. When set, the contact 'normally open' relay closes and, therefore, shall perform the function of the button 'set', that is, keeps the coil energized and therefore guarantees that the state will remain so (1) until the button "reset" is pressed, de-energizing the coil and causing the memory to another state (0).
Simple and functional.
Have fun trying to build a 4-bit memory!