Volatile memory
Beginner
Memory that forgets everything when the power goes off.
Novice
Memory that needs power to keep its data. SRAM and DRAM are volatile; flash is non-volatile.
Expert
Volatility is a property of the storage mechanism: a latch (SRAM) or a leaking capacitor (DRAM) versus charge isolated by thick insulators (flash). Non-volatility costs write speed, write energy and endurance.
Explained in Memory cells (Transistors).
See also: SRAM (static RAM), DRAM, Flash memory.