A worm is a self-replicating computer program, which sends copies of itself to other computers on the same network without any user intervention, taking advantage of security shortcomings on the target computer. Unlike a virus,a worm does not need to attach itself to an existing program. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on the target computer.
Worms are often carried inside another file, usually a Word document or an Excel spreadsheet. A worm will usually release a document that already has the worm macro inside it. This document travels from system to system, so the entire document could be regarded as a worm.
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.