Virus, Worm, and Trojans are often used interchangeably but they are not the same.
Viruses, worms and Trojan Horses are all malicious programs that can cause damage to your computer. But there are major differences among the three.
The word “Virus” means a harmful or corrupting influence. Similarly, in computers, a Virus is something that spreads its harmful activities from computer to computers. This is done by attaching itself to some executable files of a program. A virus cannot affect your computer unless the infected file itself is executed. Thus it can remain Dormant for a lifetime.
A worm is also harmful, but unlike Virus, they don’t stay Dormant. Their purpose is to spread and replicate from one computer to another through the file transport systems or even the network. Worms can even replicate themselves on the host computer.
Many worms that have been created are designed only to spread and do not attempt to change the systems they pass through. The replicated worms consume too much system resources like bandwidth, disk or even processor causing Web servers, network servers, and individual computers to stop responding.
Any code designed to do more than spreading the worm is typically referred to as the “payload”.
Typical malicious payloads might delete files on a host system (e.g., the ExploreZip worm), encrypt files in a ransomware attack, or exfiltrate data such as confidential documents or passwords.
Trojan / Trojan horse
Trojan horse, or Trojan, is any malicious computer program which misleads users of its true intent like the deceptive wooden horse in Troy it was named after. They do damage after being installed on the computer when a user thinks they are downloading a legit software from legit source. But they are not (Yeah its possible).
Some Trojans are designed to be more annoying than malicious (like those annoying pop-ups ). Or they can cause damage by deleting files and destroying information on your system.
Trojans are also known to create a backdoor on your computer that gives malicious users access to your system, possibly allowing confidential or personal information to be compromised. Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate.