Read Only Memory
Read Only Memory (ROM) is contrasted with Random Access Memory (RAM), which is not a permanent storage medium due to its volatility (RAM, with the exception of the "nonvolatile RAM" variety, loses its contents when the power is shut off). ROM consists of electronic chips that contain a particular sequence of bits permanently burned into them. Usually a computer system's basic firmware is contained in a ROM, which gives the computer the information it needs for basic operations even before you complete a "boot" of its operating system from disk or other media. These days, most of the operating system and other programs are not on ROM, allowing them to be updated with new versions as they are released, but a "boot loader" needs to be in ROM to tell the computer how to start loading the rest of the system ("booting" comes from a reference to "pulling itself up by its own bootstraps"). In the old days, when computers didn't usually have hard disk drives, or even floppy disk drives, much more of their system software was built into ROM, often including a BASIC language interpreter.
