A worm is similar in many ways to a Virus but with one key difference: a worm does not require a host file to replicate itself. A payload-less worm usually only seeks to replicate itself across a network, they can however still cause harm by consuming extra bandwidth on a network. Much like their virus counterpart it’s when some "Payload" code is added that worms become much more dangerous.
Possibly the most common use for payload laden worms is the creation of a "backdoor" in a system which can then be used to create a "Zombie" or "Bot" computer. Networks of "Zombies" or "BotNets" are often used by spam senders or for other nefarious activities which require the power of a network.