A worm is an independent program that replicates itself from system to system without the use of any host file. The difference between a worm and a virus is that the worm does not require the use of an infected host file, while the virus does require the spreading of an infected host file. Worms generally exist inside of other files, often Word or Excel documents. However, worms use the host file differently from viruses. Usually the worm releases a document that has the "worm" macro inside the document. The entire document spreads from computer to computer, so the entire document is, in essence, the worm.
A logic bomb is a malicious program that is activated when some particular condition occurs (e.g., could be a date, or system operation).
A virus is a program that alters the way another computer operates, but it is not an independent program that can reproduce itself throughout the network. A virus spreads by inserting copies of itself into the executable code or documents.
A Trojan horse is an independent program that is disguised as legitimate software. They may look useful or interesting to an unsuspecting user, but are actually harmful when executed.
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