Trojan viruses are a type of malware that disguises itself as a legitimate software to trick users into downloading and installing it. Once the Trojan is installed, it can give hackers unauthorized access to the user's system, steal sensitive information, or cause damage to the computer. Unlike viruses, Trojans do not replicate themselves, but they can be just as harmful. They are named after the Trojan Horse from Greek mythology, where soldiers hid inside a large wooden horse to sneak into the city of Troy.