Worms
A computer worm is "a program which copies itself across a network." Worms are different from viruses in that viruses require a host program. While worms will often hide in files, they can operate entirely independently.
Worms carry out a number of different functions, and so different security techniques are necessary to protect against different types of worms. Some worms spread through vulnerabilites in network servers. Tech FAQ suggests that the best way to protect against computer worms is to keep "up-to-date in installing patches provided by operating system and application vendors." Some computer worms spread like trojan horses. The best protection against these worms is to avoid downloading suspicious e-mail attachments. .EXE files are very likely infected, but even seemingly benign Microsoft Word and Microsoft Excel files can contain dangerous macros.
Resources
"Network Worms." Viruslist. Accessed April 15, 2007. <http://www.viruslist.com/en/virusesdescribed?chapter=152540408>
Tech FAQ. "What is a Computer Worm?" Accessed April 15, 2007. <http://www.tech-faq.com/computer-worm-virus.shtml>
Baratz, Adam & McLaughlin, Charles. "Malware: what it is and how to prevent it." Ars Technia. November 11, 2004. <http://arstechnica.com/articles/paedia/malware.ars/>