(Network) wormA type of malicious computer program, mostly designed to spread itself via networks.
Primairily: worms are designed to replicate themselves, they don't necessarily contain harmful payloads, although most of them do. It scans the network for vulnerable machines using the computer where it's on as the host-machine. Typically worms spread exponentially because all infected machines show the same behaviour, this often causes network-overload problems.
The major two differences between a worm and a virus are that a worm is a standalone application that does not need a host program where viruses always do, second: viruses cause harm to (system) files where worms show this behaviour only sometimes but not always.
Worms can easily spread through shared folders, e-mails, malicious web pages, and servers with a large number of vulnerabilities in the network. Some worms collect data, other worms install backdoors or other malicious software, but in all cases a worm can be considered malware because they operate without user-consent.