Worm

Taken from WhatIsMyIP.com

2. Definition: A worm is another replicating malware carrying a payload. The difference between a virus and a worm is that a worm doesn't need to attach itself to an existing executable or file. Worms can multiply themselves and can propagate out of the networks, infecting other networks and other systems, thus creating huge havoc or damages.
   
   Since a worm doesn't depend on a carrier, it doesn't need to have a user activate it: it activates without human interaction.
   
   Reason for the name: Similarly to viruses, worms are parasitic creatures that infect an organism without attaching themselves to cells.
   Propagation: The same way as viruses, by launching the payload software that it carries. The difference is that there is no human intervention. Worms propagate by finding and infecting vulnerable hosts.
   Examples of well-known worms: Melissa, Morris, ILOVEYOU, and W32.Stuxnet.