A Trojan horse is a type of malware that disguises itself as a legitimate software application to trick users into downloading and installing it. Unlike viruses or worms, Trojans do not replicate themselves; instead, they rely on users to execute them. Once activated, they can perform various malicious actions, such as stealing personal information or granting unauthorized access to the user's system. Trojans can be delivered through various means, including email attachments, infected software downloads, or compromised websites. Users are often unaware of the threat until it is too late, making it essential to maintain updated security software and practice safe browsing habits to avoid infection.