Memory that stores data long-term. This memory is non-volatile, meaning it keeps data even when the device is powered off.
This memory stores user content, as well as a copy of all software so it can be loaded into RAM when needed.
ROM is better known as flash memory.
See: Flash Memory
"Read only" is actually a misnomer that has carried over from older devices that had an OS and applications that were generally fixed and therefore stored in truly "read-only" memory. Modern devices use storage memory that easily and frequently changed, but the term has remained.