A 木馬 (Trojan) is a type of malicious software that disguises itself as a legitimate program to trick users into downloading and installing it. Once activated, it can create backdoors for hackers, allowing them to access the infected system without the user's knowledge. Unlike viruses or worms, which can replicate themselves, a 木馬 relies on user interaction to spread. It often appears in the form of email attachments, software downloads, or links, making it essential for users to be cautious and verify the source before opening files or clicking on links.