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.
Armageddon is a Backdoor Trojan that infects Windows 9x. When run it can give unlimited access to a system to anyone running the appropriate client software. The Trojan was discovered in France. To work, it requires components be gathered from different web pages.