Computer worms are programs that reproduce, execute independently, and travel across network connections. The key difference between a virus and worm is the manner in which each reproduces and spreads. A virus is dependent upon the host file or boot sector and the transfer of files between computers to spread, whereas a computer worm can execute completely independently and spread on its own accord through network connections.
The security threat from worms is equivalent to that of viruses. Computer worms are skilled enough to do serious damage such as destroying crucial files in a system, slowing a system down to a large degree, or even causing some critical programs to stop working. Two very prominent examples of worms are the MS-Blaster and Sasser worms.