What is a TROJAN and where did the name come from?

The term Trojan refers to the wooden horse used by the Greeks to sneak inside the city of Troy and capture it. The classic definition of a Trojan is a program that poses as legitimate software but when launched will do something harmful. Trojans can't spread by themselves, which is what distinguishes them from viruses and worms.

Today, Trojans are typically installed secretly and deliver their malicious payload without your knowledge. Much of today’s crimeware is comprised of different types of Trojans, all of which are purpose-built to carry out a specific malicious function. The most common are Backdoor Trojans (often they include a keylogger), Trojan Spies, password stealing Trojans and Trojan Proxies that convert your computer into a spam distribution machine.