A computer worm is a program containing malicious code that attacks host computers and spreads via a network. The basic difference between a virus and a worm is that worms have the ability to replicate and travel by themselves. They are not dependent on host files (or boot sectors).
Worms proliferate by means of email or network packets. In this regard, worms can be categorized two ways:

• email – distributing themselves to email addresses found in a user’s contact list and

• network - exploiting security vulnerabilities in various applications.

Worms are therefore much more viable than computer viruses. Due to the availability of the Internet, they can spread across the globe within hours of their release – in some cases, even in minutes. This ability to replicate independently and rapidly makes them more dangerous than other types of malware, such as viruses.
A worm activated in a system can cause a number of inconveniences: it can delete files, degrade system performance, or even deactivate some programs. Its nature qualifies it to serve as a "means of transport" for other types of infiltrations.
If your computer is infected with a computer worm, it is recommended that you delete infected files, because they likely contain malicious code.

Examples of well-known worms are: Lovsan/Blaster, Stration/Warezov, Bagle, and Netsky.