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.