Named after the Trojan Horse of ancient Greek history, a

trojan is a network software application designed to remain

hidden on an installed computer. Trojans generally serve

maliciious purposes and are therefore a form of malware,

like viruses.

Trojans sometimes, for example, access personal information

stored locally on home or business computers, then send

these data to a remote party via the Internet.

Alternatively, trojans may serve merely as a "backdoor"

application, opening network ports to allow other network

applications access to that computer. Trojans are also

capable of launching Denial of Service (DoS) attacks. A

combination of firewalls and antivirus software protect

networks against trojans.

Trojans are similiar to worms. In contrast to worms and

viruses, however, trojans do not replicate themselves or

seek to infect other systems once installed on a computer.