DRAM, or dynamic random-access memory, is the fast memory that a processor works in. It is the "8 GB RAM" printed on a phone's box, and the RAM in every laptop and server. DRAM loses everything when the power goes off. So it holds only what the device is working on right now. Files that must be kept are stored in flash memory.
Each bit in DRAM is stored in a tiny cell made of one transistor, a microscopic electrical switch, and one capacitor, a tiny store of electric charge. A charged capacitor stands for 1, and an empty one for 0. The charge leaks away within a fraction of a second. So the memory reads and rewrites every cell about every 64 milliseconds. This rewriting is called refresh, and it is why the memory is called dynamic. Random access means that the processor can read any cell directly, in a few tens of nanoseconds, without going through the cells before it. A nanosecond is a billionth of a second. An 8 GB phone holds about 6,400 crore such cells.
DRAM comes in different forms for different machines. Phones use a low-power kind called LPDDR, mounted right beside or on top of the system-on-chip. Servers use memory sticks, called DIMMs, placed next to the CPU. AI processors use high-bandwidth memory (HBM), in which DRAM chips are stacked on top of each other beside the processor.
Consider a phone with 8 GB of RAM, owned by a shopkeeper in Patna. He has a UPI app, WhatsApp, the camera and a cricket streaming app open. The operating system, Android, and its services take about 3 GB. The open apps take most of the rest. Then he opens a game that needs another 2 GB. There is not enough free RAM. So Android closes the app he used least recently, the camera, and frees its memory. When he later returns to the camera, the phone must load it again from flash memory, which takes a second or two instead of being instant. This is why a phone with more RAM keeps more apps open at once.
A processor often has to wait for DRAM, because it can compute faster than memory can supply data. This gap is called the memory wall.
Only three companies make DRAM. A modern memory factory costs more than ₹1 lakh crore, so few companies can afford one. Memory chips from different makers are very similar, so buyers choose mainly on price. Prices rise and fall sharply as supply and demand change, which is explained on memory chip cycle. India does not yet make DRAM. Micron is building a chip assembly and test plant at Sanand in Gujarat, which will package and test memory chips made abroad.