A Trojan Horse, or Trojan, is a kind of malicious software that presents itself as a benign or mundane action such as an ad worth clicking or file to be downloaded.
Trojans combine social engineering with the introduction of a certain kind of malware. As in the Trojan War myth, a computer user is similarly duped into ushering in adversaries. They download an email attachment or click an ad that either presents an offer or seems risk-free. Nowadays administrative privileges usually entail user warnings before such actions, however the social engineering component makes trojans persist.
A trojan's intent is generally different than that of other forms of viruses or malware, which tend to be self-propagating to amplify damage regardless of their being revealed. Trojans aim to remain hidden. They generally open a backdoor through which the computer can be accessed externally by a hacker, or correspond with the hacker who developed it to send sensitive information from the user's device.
"My computer has been overtaken by malware and admins think, after speaking with me, that I may have inadvertently downloaded a trojan that's giving an unauthorized party control my device. My laptop could be participating in a botnet right now."