Viruses, Worms & Trojans
A computer virus is a computer program usually hidden within another program that produces copies of itself and inserts them into other programs or files. They usually perform a malicious activity such as deleted your files. Computer viruses are always made by humans, and don’t occur on their own.
Worms are very similar to viruses, in that they are computer programs that replicate functional copies of themselves. They also, but not always, contain some functionality that will interfere with the normal use of a computer or a program. Unlike viruses, worms exist as separate entities; they do not attach themselves to other files or programs. Because of their similarity to viruses, worms also are often referred to as viruses.
A Trojan horse is a program that does something undocumented which the programmer intended, but that users would not accept if they knew about it. By some definitions, a virus is a particular case of a Trojan horse, namely, one which is able to spread to other programs (i.e., it turns them into Trojans too). According to others, a virus that does not do any deliberate damage (other than merely replicating) is not a Trojan. Finally, despite the definitions, many people use the term “Trojan” to refer only to a non-replicating malicious program.
Below is a video on the difference between Computer Viruses, Worms, and Trojans:
15 of the Worst Computer Viruses in History: