A worm is actually a bit different from a virus as unlike viruses, they do not have to attach themselves to a computer program. You can consider it to be a sub class of virus. Its a self replicating software program and when the infected computer is attatched to a netwok, the worm will replicate and send copies to other computers on the network and there main purpose is to spread across all the computers in a network. It can travel without any human help by making use of the information transport features in the computer. At times, the author of the worm creates a network of infected computers(zombie computers) to which he has access due to the worm and then use this network to send spam mail. They damage the network but a few can delete files on your computers as well. They damage the network by consuming a lot of bandwidth because millions of messages ar being sent as spam from that computer. It also consumes a lot of the computer’s memory and it will stop responding. The first ever worm was the Morris worm and was written by a student of the Cornell university.