Final answer:
A program that looks legitimate but performs unwanted activities when activated is called a Trojan horse virus. It differs from worms, adware, and spyware, presenting itself as useful while intending to cause harm or theft of information.
Step-by-step explanation:
A program that appears legitimate, but executes an unwanted activity when activated is known as a Trojan horse virus. Unlike a worm, which is a self-replicating program that spreads to other computers, a Trojan horse presents itself as useful software while hiding its true malicious intent. Once installed and activated, it can perform a variety of actions, such as stealing information, taking control of the user's system, or harming the system's functions.
The term 'virus' is often used in a general sense to describe all kinds of malware, but a Trojan horse is actually a specific type of malware that must be executed by the user, usually by way of deception. It's important to note that a Trojan is different from others like adware, which is designed to show unwanted advertisement, or spyware, that secretly collects user information without consent.