Definitions

What is a Virus?
A virus is a manmade program or piece of code that causes an unexpected, usually negative, event. Viruses are often disguised games or images with clever marketing titles such as "Me, nude."
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A computer virus is a piece of code that is secretly introduced into a system in order to corrupt it or destroy data. Often viruses are hidden in other programs or documents and when opened, the virus is let loose.

What is a Worm?
Computer Worms are viruses that reside in the active memory of a computer and duplicate themselves. They may send copies of themselves to other computers, such as through email or Internet Relay Chat (IRC).
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A computer worm is a program which copies itself across a network.A computer worm differs from a computer virus in that a computer worm can run itself. A virus needs a host program to run, and the virus code runs as part of the host program. A computer worm can spread without a host program, although some modern computer worms also use files to hide inside.

What is a Trojan Horse?
A Trojan horse program is a malicious program that pretends to be a benign application; a Trojan horse program purposefully does something the user does not expect. Trojans are not viruses since they do not replicate, but Trojan horse programs can be just as destructive.
Many people use the term to refer only to non-replicating malicious programs, thus making a distinction between Trojans and viruses.
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This is a program that disguises itself as another program. Similar to a virus, these programs are hidden and cause an unwanted effect. They differ from viruses because they are normally not designed to replicate like a virus.