Worm vs Virus
Computer worm is a harmful program that differs from viruses by several points.
First, the worm is a standalone program that can be found on the hard drive , unlike viruses that lurk like parasites in files or in the executable code in the boot sector of the disk. A worm can arrive directly by the network by taking advantage of an open port, but the most traditional method is to introduce as an attachment attached to an email. But most of the time you have to click on the attachment so that the worm runs.
A worm does not grow locally, unlike viruses but the most usual method of spreading worm is to send in automatically generated emails. These mails are sent to the knowledge of the user to various addresses. These addresses are usually obtained by the worm in the files on the disk (especially the address book). Worms usually install on the computer other harmful programs: spyware, key loggers, Trojans. These programs can be exploited to spy on your activity, capture passwords or credit card numbers, or take control of the remote computer to turn it into PC zombie. Such a PC can be used as relay attacks denial of service or for sending mass spam.