Malware (malicious software) is a general term used to describe any program designed to cause harm. Some common types of malware include viruses, worms and trojans.
A virus is a malicious program that attaches itself to and “infects” other software applications and files, disrupting computer operations. Viruses often carry a “payload,” which is an executable script designed to damage, delete or steal information from a computer.
A virus is a self-replicating program, meaning it copies itself. Typically, a virus only infects a computer and begins replicating when the user executes the program or opens an infected file.
Viruses spread from computer to computer only when users unknowingly share infected files. For example, viruses can spread when users send emails with infected documents attached.
A worm is similar to a virus but with an additional dangerous element. Like a virus, a worm can make copies of itself, but it does not require a person to send it along to other computers. A worm spreads rapidly across a network without having to attach itself to another program.
Since worms are so quick and pervasive through a network, they quickly absorb resources and can bring not just one computer down, but thousands, potentially shutting down an entire network.
A trojan is a malicious program disguised or hidden within another program that appears to be safe (much like the myth of the Trojan horse). When a trojan is executed, it allows attackers to gain unauthorized access to the computer in order to steal information and cause harm. Trojans commonly spread through email attachments and Internet downloads.