Trojans are malicious applications that pose themselves as legitimate software in order to trick users to install them. Once on the victim's machine, it may run any number of malicious process to steal vital information or inflict damage to other software.
The virus code, "Casus" registers itself in the system registry so that it will automatically run when the infected system is rebooted. Via e-mail and ICQ, Casus sends notifications to its host (hacker receiving the network notifications) and begins to listen, over the TCP/IP protocol on port 21. Having received the notices, informing him of located networks, the virus host with the help of any FTP-client gains access to file systems on victim computers.