UDP Question:

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.