Worms

Worms can replicate themselves from computer to computer without infecting a host program, unlike viruses, which must be carried by a host. Although worms are often carried inside another file, usually a Word document or an Excel spreadsheet, they use the host in a different way from a virus.
A worm will usually release a document that already has the worm macro inside it. The entire document travels from system to system, so the entire document should be regarded as a worm. W32.Mydoom.AX@mm is an example of a worm. It was designed to allow spammers to send spam e-mail from infected computers
Like Trojans, worms are self contained programs that are designed to copy themselves from computer to computer. Different worms exploit different weaknesses in the programs that allow computers to communicate with each other.
A common purpose of worms is to install a backdoor into a computer. These are programs that allow others to gain access to your computer.
You can find out more about the differences between viruses, Trojans and worms by visiting the following web site: Viruses, Trojans and Worms.