Trojan malware is something that pretends to be something else to mask its true nature of gaining access to your computer for ill intent. For example, you might think you’re downloading and installing a fun game, when really you are downloading malware hidden inside the game.
The user themselves initiates the download and installation of Trojan malware.
Antivirus may be able to catch the malware when it runs. However, sophisticate Trojan malware may be able to disable antivirus programs before it runs.