What is a Computer Worm?

A computer worm is a self-replicating computer program that penetrates an operating system with the intent of spreading malicious code. Worms utilize networks to send copies of the original code to other computers, causing harm by consuming bandwidth or possibly deleting files or sending documents via email. Worms can also install backdoors on computers.

Example:

• Melissa
• ILOVEYOU

Source: http://www.pctools.com/

What is Polymorphic Virus?

They encode or encrypt themselves in a different way every time they infect your computer. They use different encryption and algorithms. This makes it difficult for the antivirus software to locate them using signature or string searches (since they are very different in each encryption).

Example:

• Marburg
• tuareg
• Satan bug
• elkern

Source: http://typeslist.com/