A Trojan Horse or back door is a computer program that appears to be useful, but also has a hidden and usually
malicious function that evades security mechanisms, typically by exploiting the user’s information, credential etc.
It is typically delivered through an attachment to an
email message or a link from within the email message.
The purpose of the Trojan is to steal information,
credentials, and identities for evil purposes or monitory
gain.
Advance Trojans that can and due remain undetected for
long periods of time are referred Advanced Persistent
Threats (APT’s).