A virus is an unwanted program that, in almost all cases, the user accidentally installs onto his or her computer. Some are relatively harmless and cause little, if any, damage. Some are much more hazardous and can render the information on your computer's hard drive useless.
So why do people make viruses?
Worms and Viruses
I got a question the other day from a reader wanting to know what the difference between a worm and a virus. So, without further adieu, here we go!
Viruses are self-replicating programs that embed themselves into other programs, or even the operating system , and use the host to carry out its function. Unless it attaches itself to an email, a virus stays put on the victim's computer.
A worm is also a computer program that makes copies of itself. They spread from computer to computer (instead of file to file), infecting whole systems, using up resources in the affected computer, and causing heaps of damage. Worms penetrate the computer's memory from a network, find network addresses of other computers, then send their own copies to these addresses. Once launched, a worm can email itself to everyone in an address book or launch a trojan horse.
So, worms are designed to spread out to many computers while viruses generally stay put. Worms take over your computer while viruses destroy files.
Regardless of the differences, though, they are both destructive to your system.