What’s a Trojan Horse virus?

What’s a Trojan Horse virus?: "The Visa description continues with, 'A Trojan Horse is an email virus usually released by an email attachment.' Not so. A Trojan may be sent as an attachment in email, but it's certainly not an email virus. (In fact there are few true email viruses, but that's a whole other topic). So it may or may not arrive in email, and it's equally likely to have been downloaded from a website or resulted from a P2P file transfer. In other words, vector has nothing to do with whether something is or isn't a Trojan."