A Trojan, or Trojan horse, is a type of malware that disguises itself as a legitimate, harmless program or file to trick users into downloading and installing it. Once activated, a Trojan can perform a range of malicious activities, including giving attackers unauthorized access to the infected system. This can lead to the theft of sensitive information, such as credit card numbers and passwords, and can also allow the attacker to install additional malware, potentially leading to further damage, such as the erasure of data. Unlike viruses and worms, Trojans do not replicate themselves but rely on the deception of users to spread.