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.
Alexandra is a trojan, which is spread via links in IRC chat. When run on the target PC, a new folder is created in the System32 Folder which contains configuration files for mIRC (a popular IRC chat client). The infected machine then joins a Botnet, and awaits commands from the Botnet owner.