Computer Worms are programs that self-reproduce, execute independently and travel across the network connections. The key difference between a Virus and Worm is the manner in which it reproduces and spreads. A Virus is dependent upon the host file or boot sector, and the transfer of files between computers to spread, whereas a Computer Worm can execute completely independently and spread on its own accord through network connections. 
Speed of a Worm's replication
Computer Worms can replicate in great volume. For example, a Worm can send out copies of itself to every contact in your e-mail address book, and then it can send itself to all of the contacts your contact's e-mail address books, and his/her contacts' and so on. Other Computer Worms clog networks and can cause long waits for you (and everyone else) to view web sites on the Internet. 
Types of Computer Worms
Although there are many more, the main types of Computer Worms are as follows: 
This type of Worm spreads via infected email messages. Any form of attachment or link in an email may contain a link to an compromised or infected website. In the first case activation starts when the user clicks on the attachment while in the second case the activation starts when clicking the link in the email.
Instant Messaging Worms
This type of Worm spreads via instant messaging applications by sending links to compromised or infected websites to everyone on the contact list. These are very similar to email Worms, the only difference being the way chosen to send the links.
These Worms ones will scan all available network resources using local operating system services and/or scan the Internet for vulnerable machines. An attempt will be made to connect to these machines and gain full access to them.
Protecting Yourself from Computer Worms
Ways to protect yourself and your computer from Computer Worms are: 
- Use a firewall
- Update your operating system and the software that you use (Use Windows Update to update all Microsoft products automatically)
- Use Anti-virus and Anti-spyware software
- Be careful with e-mail attachments and links on Web sites
- Use a standard user account instead of an administrator account