Even though a trojan horse is sometimes also called Trojan, it's more a
Greek. The Greeks build the so-called 'trojan horse' in the fight about Troja
to get into the town, so they are the real snoops ;)
A trojan is a program that has gotten onto your machine without your
knowledge and contains malicious code, that would for example allows persons
using another computer to connect to yours over a network. Typical trojans are
open to anyone trying to connect (any person on your local network or even the
internet). Special trojans are designed to make you machine accessible just
to the person who infected your computer with the trojan.
The access an outsider can gain using a trojan on your machine can be
nearly anything. From watching all your behaviour (like a
keylogger) to manipulating your computer to basically doing anything you
can also do using your keyboard and mouse.
Your computer can get infected with a trojan by multiple ways. A person
with physical access to your machine can place it there, but you can also
accidentally install it yourself by opening an unknown email attachment that
by chance contains a trojan.
According to some definitions, trojans are also programs that sneak into
other programs, for example to gain access. As these programs are
consciously used by the other, they are not trojans, but are backdoors.
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