Malware is software designed to cause harm to a computer and user. Some forms of malware “spy” on user Internet traffic. Examples include spyware and adware. Spyware monitors a user’s location and if enabled, it can capture sensitive information, e.g., credit card numbers, promoting identity theft. Adware also acquires user information, which is shared with advertisers and then integrated with unwanted, triggered pop-up ads.
Worms and viruses behave differently, as they can quickly proliferate and undermine an entire computer system. They also may perform unsavory activities from a user’s computer without the user’s knowledge. In the wake of a virus or worm, a computer system can experience significant damage.
– definition from Techopedia