DRAM
/ DEE-ram /
Imagine storing each bit as a little bucket of water: a full bucket means 1, an empty bucket means 0. Buckets are cheap and you can pack millions of them in a small space — but they have a slow leak, so a full bucket slowly drains, and unless someone walks the rows topping up the full ones every few milliseconds, your 1s quietly fade into 0s. DRAM is memory built from exactly such leaky buckets, which is why it is dense and cheap but must be constantly refreshed.
Concretely, a DRAM cell is astonishingly simple: one transistor and one capacitor per bit (a 1T1C cell). The capacitor holds a tiny charge that represents the bit, and the transistor is a gate that connects the capacitor to a wire when you want to read or write. This is far smaller than SRAM's six transistors, so you fit billions of bits on a chip cheaply. The two catches: the charge leaks away (so the whole array must be refreshed periodically), and reading is destructive — sensing the tiny charge drains the capacitor, so every read must be followed by writing the value back.
Why it matters: DRAM is what 'RAM' or 'main memory' almost always means — the multi-gigabyte pool a computer uses for running programs and their data. It exists because SRAM, while faster, is far too expensive and bulky to provide gigabytes. The honest trade-off is at the heart of computer design: DRAM is dense and cheap but slow, and its slowness relative to the CPU is the 'memory wall' that dominates performance for huge classes of real workloads.
A 16 GiB DRAM stick stores roughly 1.3 x 10^11 bits, each one a single tiny capacitor. Every few milliseconds the chip quietly reads and rewrites every row to keep those capacitors from forgetting — work that happens without the program ever knowing.
One transistor + one capacitor per bit makes DRAM dense — at the cost of leaks, refresh, and destructive reads.
DRAM is volatile: cut the power and the charge is gone within moments. Its 'dynamic' name comes from needing constant refresh, not from any ability to retain data without power — for that you need non-volatile memory.