A computer worm is a malicious program that is self-replicating and self-executing. It typically makes use of a network to circulate duplicate copies of itself to other devices on the network.
In contrast with a computer virus, a worm need not hide in an existing program. Rather than requiring user action to steal, infect, or destroy files, a worm will connect and if possible, break in to other computers which its host computer has access to. Like the Invertebrate it is named after, it could be said to crawl about and reproduce with no external assistance, rarely being noticed. Worms are used as a delivery method form other malware, such as spyware and viruses.