The word "worm" can mean different things, depending on the context. It most frequently refers to a type of computer virus. It may also refer to a type of write-once data storage.
A worm is a variety of computer virus, similar to a Trojan Horse virus, that replicates itself without any human interaction. Once a worm has infected a host computer, it typically scans the local network for other computers with a particular vulnerability; when it finds one, it exploits that vulnerability to infect that computer, then scans the network again for another computer to infect.
Worms automatically spread themselves over a network when activated, but they can initially infect a computer using the same tricks other kinds of viruses do. They can spread using a file sent as a suspicious email attachment or file download. These files don't need to be executable files to spread a worm; hackers have found vulnerabilities in other file formats that allow them to embed malicious code in images, documents, and video and audio files. Worms can also spread online via code in HTML web pages that exploit vulnerabilities in web browsers.
While this self-replicating behavior is what defines a worm, the other effects of a worm vary from worm to worm. Many worms are designed only to spread themselves; these worms cause computer systems and networks to slow down by gradually filling up storage space and memory and overloading network bandwidth. Other worms act as a way to spread a payload, carrying another virus that could delete files, encrypt files to hold for ransom, install spyware, or create a backdoor to allow remote control of a computer.
WORM (stands for "Write Once, Read Many") is a broad term for data storage media that can be written to once, then becomes read-only. Using a WORM data storage method ensures that the data written to the medium remains unaltered.
Early WORM data storage devices used writable optical discs stored in a protective cartridge. These drives were expensive and very large (with cartridges 12 inches or more in diameter) but had relatively high storage capacity and were useful for archiving data. They were eventually made obsolete by the introduction of writable CD-R discs; writable DVD-R and DVD+R discs are now the most commonly-used form of WORM media.