A worm is a program that copies itself to another computer (independently executable). A worm can replicate itself without any human interaction, and it does not need to attach itself to a software program in order to cause damage.
Computer worms can arrive as attachments in spam emails or instant messages. Once opened, these files can provide a link to a malicious website or automatically download the computer worm.
Worms can modify and delete files, and they can even inject additional malicious software onto a computer. Sometimes a computer worm’s purpose is only to make copies of itself over and over — to overload a device or network. In addition to wreaking havoc on a computer’s resources, worms can also steal data and allow a hacker to gain control over a computer and its system settings.