Off/on switch patterns in ROM are preset and fixed in position by the manufacturer. Some ROM chips contain basic startup instructions that are activated each time you turn on the computer. Other ROM chips contain instructions for operating certain devices, like your monitor. These instructions are put into ROM because it's quicker than having to go out and get them from the disk every time they're needed.

Read Only
The computer can "read" the switch settings in ROM but can't "write" to it. Hence, any instructions and data stored inside are permanent--they cannot be changed or erased.
 

ROM vs. Disk
ROM chips are similar to disk media. You can touch and hold them (hardware), yet they contain instructions and data (software). Some portable computers come with software preloaded to ROM, eliminating the need for program disks. However, the advantage of programs on disk is that a new version of the program can be inserted into the computer and used immediately. With ROM, the computer case must be opened to replace the old ROM chip with a new one--something most users are reluctant to do.

ROM vs. CD-ROM
ROM is not the same as CD-ROM (Compact Disc-Read Only Memory). A CD-ROM is a foil-coated plastic disc spun in a CD-ROM drive and used for storing large volumes of instructions/data, especially music and video. CD-ROM is called Read Only because it can not be erased or rewritten. However, it more appropriately should have been named CD-ROS, that is, Read Only Storage.

PROM (prawm)
A ROM chip starts as a PROM or Programmable ROM. Once programmed, it cannot be altered.