CD-ROM stands for Compact Disc Read Only Memory. In practice, this refers to CDs that have been manufactured by having the CD pits stamped directly onto the disc surface.
This type of optical disk is capable of storing large amounts of data -- up to 1GB, although the most common size is 700MB. 1GB is usually achived by specialised recorders, and readers, such as the one used on the Sega Dreamcast.
CD-ROMs are stamped by the vendor, and once stamped, they cannot be erased and filled with new data. To read a CD, you need a CD-ROM player. All CD-ROMs conform to a standard size and format, so you can load any type of CD-ROM into any CD-ROM player. In addition, CD-ROM players are capable of playing Audio CDs, which share the same technology.
CD-ROMs are particularly well-suited to information that requires large storage capacity. This includes large software applications that support color, graphics, sound, and especially video.
The CD-ROM is largely being replaced by DVD-ROM, which allows for greater capacities.