Perhaps the most common way today is via e-mail. Known as mass mailers due to the technique of spamming themselves to every address in the e-mail address book, such worms generally require a person's action to spread. Typically, that means opening an attachment in the infected e-mail. By scamming people, such mass-mailing worms are similar to Trojan horses. A good example of a mass mailer is the recent AnnaKournikova worm.
Some worms squirm into another computer through security holes.
Whether taking advantage of unprotected, shared drives or of a vulnerability in FTP software, such network-aware worms don't require a person's action to spread. A strict definition of a Trojan horse is any program that does something besides what a person believes it will do. Modified programs that open a back door into a system or a program hidden inside of a humorous animation are typical examples of Trojan horses. Yet some have broadened the definition to include commercial software that collects data on the person running the program and sends it back to the company without adequate warning to that person. Many mass-mailing worms are considered Trojan horses because they have to convince someone to open them.