A worm is a program or algorithm that replicates itself over a computer network and usually performs malicious actions, such as using up the computer's resources and possibly shutting the system down. 
The computer resources that worms use are computer time and network bandwidth when they replicate, and often carry payloads, (a computer program delivered by encapsulation) that do considerable damage.
According to tech-faq.com, “A computer worm differs from a computer virus in that a computer worm can run itself. A virus needs a host program to run, and the virus code runs as part of the host program. A computer worm can spread without a host program, although some modern computer worms also use files to hide inside.”