A trojan is a malicious application that appears to do one thing, but actually does another. Like it?s name sake, the mythical Trojan Horse, malicious code is hidden in a program or file that appears useful, interesting, or harmless. Popular examples are video codecs that some sites require to view online videos. When the codec is installed, it may also install spyware or other malicious software.
A backdoor trojan differs from a trojan in that it also opens a backdoor to your system. They?re also sometimes call Remote Access Trojans (RAT). These are the most widespread and also the most dangerous type of trojan. They are so dangerous because the have the potential to allow remote adminstration of your system. As if a hacker were sitting at your keyboard, only worse. There?s almost no limit to what they can do.
continued @ [Geeks to Go]