CD-ROM stands for "compact disc read-only memory." CD-ROMs are optical compact discs which are commonly used to store data and transfer it across physical locations from reading site to reading site.
A CD-ROM can store more than 500 MB of data. That is more than 25,000 pages of written material, a tremendous amount of information for such a small space. The data storage advances of the 21st century have begun to eclipse the CD-ROM, and it has grown more obsolete as time passes and data storage becomes smaller, more efficient and less easily damaged by scratching and other forms of wear.
Some CD-ROM discs are rewritable or can have new data appended to them even after data has been written. Others come with inviolable content already pressed and coded. They are not as versatile as fully rewritable storage devices like USB thumb drives and other forms of external hard drives, but they are easy to store and file.
DVD-ROM discs have much greater storage capacity than CD-ROM discs, up to 17 GB each. They are, however, prey to many of the same shortfalls as CD-ROMs. They are easily damaged, hard or impossible to reuse and in other ways ill-suited to active archival work.