You've recently noticed a change on your computer. You now see a dozen unwanted pop-up windows attempting to sell you things you didn't ask for, regardless of what you search for on your web browser. What kind of problem is your computer experiencing?

Spyware is different from other types of malware in that it actively works for a third party. Unlike self-replicating viruses and worms, spyware is spread to computers by users who unintentionally ask for it. The users sometimes don't realize they've requested it, but they have through downloading other programs, going to malicious websites, and other means. The spyware program keeps track of the user's activities and responds by displaying unwanted pop-up advertisements (often referred to as adware), compiling user data for marketers, or intercepting personal information like credit card numbers.
Software known as ransomware takes control of a computer and prevents access to the data until a ransom is paid. Computers that have been taken over by another party and are being used maliciously are called zombies. The collective behavior of numerous zombies creates a botnet. In most cases, the computer user is not aware of the attacker's existence. When a user downloads a seemingly innocent-looking software program or digital image, a Trojan is downloaded as well. The Trojan enters the computer system after it has been downloaded.