There are several different kinds of viruses. A Trojan Horse program comes with a hidden surprise intended by the programmer but totally unexpected by the user. Trojan Horses are often designed to cause damage or do something malicious to a system, but are disguised as something useful. Unlike viruses, Trojan Horses don't make copies of themselves. Like viruses, they can cause significant damage to a computer.
Worms are like viruses in that they do replicate themselves.
However, instead of spreading from file to file, they spread from computer to computer, infecting an entire network system. Viruses are either benign or malignant. The majority of viruses are harmless and do no real damage to a computer or files. A benign virus might do nothing more than display a message at a predetermined time or slow down the performance of a computer. Malignant viruses cause damage to a computer system, such as corrupting files or destroying data.