A typical CD-Rom disc stores around 650 Mb, which as much information as the expensive hard disk that is permanently fixed inside a PC. The first big difference is that CDs can be made very cheaply so they are an ideal way of distrubuting software. The second is that CD-Roms are pressed in a factory and you can no more change the information on them than you can change the pre-recorded music on an audio CD. (Rom stands for Read-only memory)
