what do worms do ? i mean, how do they affect your system?

: Computer worms are malicious software applications designed to spread via computer networks. Computer worms are one form of malware along with viruses and trojans. A person typically installs worms by inadvertently opening an email attachment or message that contains executable scripts.

Once installed on a computer, worms spontaneously generate additional email messages contaning copies of the worm. They may also open TCP ports to create networks security holes for other applications, and they may attempt to "flood" the LAN with spurious Denial of Service (DoS) data transmissions.

Being embedded inside everyday network software, computer worms easily penetrate most firewalls and other network security measures. Antivirus software applications attempt to combat worms as well as viruses.