Article about MyDoom
The new version is a mass-mailing worm that does not contain an attachment, as some earlier versions of the worm program have done. A worm is a self-replicating computer program that -- like a computer virus -- can cause damage to a computer's software by attaching itself to programs.
The new Mydoom e-mail messages direct users to click on a link, directing them to an infected machine. Following the hyperlink results in an infection occurring on the target victim's system. Some of the e-mail transmissions appear to be from PayPal, the online payment system that is part of eBay.