Worm has two widely different definitions. One refers to a computer virus and the other is an optical storage technology:
1) A type of computer virus
Just like regular worms tunnel through dirt and soil, computer worms tunnel through your computer's memory and hard drive. A computer worm is a type of virus that replicates itself, but does not alter any files on your machine. However, worms can still cause havoc by multiplying so many times that they take up all your computer's available memory or hard disk space. If a worm consumes your memory, your computer will run very slowly and possibly even crash. If the worm affects your hard disk space, your computer will take a long time to access files and you will not be able to save or create new files until the worm has been eradicated.
Worms are hard to detect because they are typically invisible files. They often go unnoticed until your computer begins to slow down or starts having other problems. Unlike viruses and Trojan horses, worms can replicate themselves and travel between systems without any action from the user. For these reasons, it is good to have an antivirus program installed on your system that can detect and remove worms before they have a chance to replicate or spread to other computers. Security updates such as Windows Update also patch security holes that allow worms to infect your computer. So keep your security updates and virus definitions up-to-date and you should be able to keep your computer worm-free.
2) An optical storage technology
WORM can also mean "Write Once, Read Many." It is an optical storage technology that allows a disc to be written only once but read an unlimited number of times. WORM devices were introduced in the 1970's and gained popularity as a way of archiving data. The storage capacity of WORM discs began around 140MB, but increased to more than 3.0GB over the past few decades. Yet the WORM technology has no standard format, so WORM discs are only compatible with the drives that wrote them. This limitation has kept WORM equipment relatively expensive and has kept the technology from gaining widespread acceptance.
Today, most optical drives are based on either CD-R or DVD-R technology. Unlike WORM discs, CD-Rs and DVD-Rs can typically be read by any CD or DVD drive, regardless of the manufacturer. Because of their improved compatibility and lower costs, writable CDs and DVDs have all but replaced WORM media.