All three are types of malicious software (malware), but they differ in their behavior and method of propagation.
A virus is a type of malware that spreads by infecting files or systems. When an infected file is opened or executed, the virus code is activated and can replicate itself by infecting other files on the same system or network. Viruses can also have a payload that can cause damage, such as deleting files, stealing data, or crashing the system. For example, the famous "ILOVEYOU" virus in 2000 infected millions of computers worldwide and caused billions of dollars in damage by overwriting and deleting files.
A worm, on the other hand, spreads by taking advantage of security vulnerabilities in networked systems. Once it infects one computer, it can automatically propagate itself to other vulnerable systems on the same network, without the need for human interaction. Worms can also have a payload that can cause damage, such as creating backdoors for remote access, stealing sensitive information, or launching DDoS attacks. For example, the Wannacry ransomware worm in 2017 exploited a vulnerability in Microsoft Windows to infect over 200,000 computers in a matter of days and encrypt their files, demanding payment in exchange for the decryption key.
A Trojan (or Trojan horse) is a type of malware that disguises itself as legitimate software, such as a game or utility program, to trick users into downloading and installing it. Once installed, the Trojan can perform a variety of malicious actions, such as stealing passwords, logging keystrokes, taking screenshots, or opening backdoors for remote access. Trojans do not typically replicate themselves like viruses or worms, but they can be used to deliver other types of malware or to create botnets for further attacks. For example, the Zeus Trojan in 2007 was capable of stealing banking credentials and other sensitive information, and was responsible for millions of dollars in fraud.
In summary, viruses, worms, and Trojans are all forms of malware, but they have different propagation methods and behaviors. Knowing the differences can help in identifying and mitigating their risks.