Trojan horse

What is Trojan horse? Why it's so important to know? Trojan horse, or trojan for short, is a term used to describe malware that appears, to the user, to perform a desirable function but, in fact, facilitates unauthorized access to the user's computer system. The term comes from the Trojan Horse story in Greek mythology. Trojan horses are not self-replicating which distinguishes them from viruses and worms. Additionally, they require interaction with a hacker to fulfill their purpose. The hacker need not be the individual responsible for distributing the Trojan horse. It is possible for hackers to scan computers on a network using a port scanner in the hope of finding one with a Trojan horse installed.
Trojan horses are designed to allow a hacker remote access to a target computer system. Once a Trojan horse has been installed on a target computer system it is possible for a hacker to access it remotely and perform various operations. The operations that a hacker can perform are limited by user privileges on the target computer system and the design of the Trojan horse.
Operations which could be performed by a hacker on a target computer system include:

1. Use of the machine as part of a Botnet (e.g. to perform Distributed Denial-of-service (DDoS) attacks)
2. Data Theft (e.g. passwords, security codes, credit card information)
3. Installation of software (including other malware)
4. Downloading of files
5. Uploading of files
6. Deletion of files
7. Modification of files
8. Keystroke logging
9. Viewing the user's screen
10. Wasting computer space, of course, because it reduplicate self in host computer.