Worms are viruses that create copies of themselves across the internet.
Worms differ from computer viruses because they can propagate themselves, rather than using a carrier program or file. They simple create exact copies of themselves and use communication between computers to spread.
The Conficker worm is an example of an internet worm that exploits a system vulnerability in order to infect machines over the network. Such worms are capable of spreading very rapidly, infecting large numbers of machines.
Some worms open a “back door” on the computer, allowing hackers to take control of it. Such computers can then be used to send spam mail (aka Zombies)
Operating systems vendors regularly issue patches to fix loopholes in their software. You should update your computer regularly by using Windows Update or selecting the Apple logo and choosing Software Updates.