Definition par
Command.com
Virus: A computer virus is a self-replicating sequence of code, contained in a legitimate program, which purposely copies itself and infects other programs. To qualify as a virus, it must self-replicate, i.e. create copies of itself to continue the infection. The program in which the virus is hidden is referred to as the host. In addition to replication, some viruses carry a “payload” or portion of the program that is designed to corrupt data or cause some other malicious occurrence.
Trojan: A Trojan Horse is a program that does something other than what it is advertised or believed to do. It is widely accepted that the distinction between a virus and a Trojan lies within its ability to self-replicate.
Worm: Unlike a virus, a worm is a self-contained program that copies itself to other computers, usually via network connections. In other words, the worm is not “hidden” inside a legitimate program – the program is the worm. However, the worm may be disguised for distribution as a legitimate program. A worm is generally designed to repeatedly copy itself in memory or on a disk drive until there is no more space left for the computer to work