RAM is the memory a computer works in. You can read any word and write any word, as often as you like, while the system runs.
The name says "random access": any address can be reached directly, in about the same time, whatever was accessed before. Compare a tape, where you must wind past everything in between. (A ROM is random-access too; "RAM" is just the historical name for read/write memory.)
To support writing, a RAM chip has a read/write control as well as a chip select. On this site R/W = 1 means read and R/W = 0 means write.
The two main kinds:
- SRAM (static): each bit in a small latch. Fast, no refresh, larger cells. Used for caches and on-chip memory.
- DRAM (dynamic): each bit as charge on a tiny capacitor. Dense and cheap, but must be refreshed. Used for main memory.
Both are volatile: switch the power off and the contents are gone. That is why every computer also needs some non volatile memory to hold the program it runs at start-up.