Worms
A worm is a self-replicating computer software. It uses a network such as the Internet to send copies of itself to other computers on the network exploiting security vulnerabilities. Opposed to a computer virus, it doesn't need to piggyback on an existing program in order to take action.
A worm can cause damage to a network by consuming bandwidth and to systems by passing through, and changing or influencing them. Worms usually carry payloads that do considerable damage. A payload is a code that is designed to perform different actions in addition to "just spreading" the worm; these include, deleting files on a host, sending documents by email and installing a backdoor on PCs.
Worms can spread throughout the Internet very quickly and sometimes these
programs can affect the entire network making it unusable.