What is Trojan?
Trojans are probably the most widely spread malware these days. As the name suggests they are something different then what they appear to be, in a malicious sense. The target of these threats is financial information, using computer resources and/or dDoS by flooding made possible by having thousands of infected computers send small bits of information to another machine. Trojans are of the following types: Clicker, Downloader, Dropper, Flooder/Nuker/Spammer, Keylogger, Startpage, Dialer.
How to protect yourself