Flash memory is solid-state nonvolatile computer memory, which can be read and written and keeps its values even with power off. A common use is for thumb drives, but there are many other devices which use it for storage. A type of "managed Flash memory" is Secure Digital (SD).
The two main types are NAND and NOR, named after the logic gates they resemble. NAND memory is written or read in blocks, so it is suitable for substituting for disk drives (which read and write in sectors). NOR memory can read or write single bytes, so it can substitute for RAM.
Before flash memory, nonvolatile memory consisted of EPROMs, which could only be rewritten in their entirety (and then behaved like ROMs until rewritten again), and EEPROMs which could be rewritten in small blocks or single bytes, but were slow.