Acorn BBC Model B - Paged ROM
The unexpanded Model B has four physical ROM sockets capable of accepting upto 16kB ROM/EPROMs on the main printed circuit board (PCB). The operating system was designed to support 16 ROMs through hardware expansion. The sockets are located at the front right of the computer tucked up under the keyboard.
Back in the '80s several manufacturers made expansion boards for the model B to increase the physical number of ROM sockets and provide extra RAM (sideways RAM). These boards were physically large and consumed a lot of power which affected the cooling in a machine notorious for becoming unstable when warm.
Software in ROM loads instantly which is a tremendous advantage for a tape only machine or even one with a floppy drive. Since the software in the ROM does not load into the Model B's main memory this leaves more of the precious space to work on user files. Especially useful for word processors or CAD programs. Examples of common ROM software are languages such as BASIC (shipped with the machine) and filing systems like DFS for floppy drives. Word processors, CAD tools, paint programs, low level tools and even games were all available in ROM.
Extra RAM, commonly known as sideways RAM, can be used to load ROM images. Some games can make use of available sideways RAM to provide an enhanced experience and some software could use it to allow work on larger files which would otherwise not fit in memory. The terms "paged ROMs", "banked memory" and "sideways ROM/RAM" are largely interchangable.