A computer WORM is a self-contained program (or set of programs), that is able to spread functional copies of itself or its segments to other computer systems (usually via network connections). Note that unlike viruses, worms do not need to attach themselves to a host program. There are two types of worms--host computer worms and network worms. Host computer worms are entirely contained in the computer they run on and use network connections only to copy themselves to other computers. Host computer worms where the original terminates itself after launching a copy on another host (so there is only one copy of the worm running somewhere on the network at any given moment), are sometimes called "rabbits." Network worms consist of multiple parts (called "segments"), each running on different machines (and possibly performing different actions) and using the network for several communication purposes. Propagating a segment from one machine to another is only one of those purposes. Network worms that have one main segment which coordinates the work of the other segments are sometimes called "octopuses." The infamous Internet Worm (perhaps covered best in "The Internet Worm Program: An Analysis," Eugene H. Spafford, Purdue Technical Report CSD- TR-823) was a host computer worm, while the Xerox PARC worms were network worms (a good starting point for these is "The Worm Programs-- Early Experience with a Distributed Computation," Communications of the ACM, 25, no.3, March 1982, pp. 172-180).
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