Trojan is an umbrella term covering many types of malware: bots, backdoor Trojans and downloader Trojans.
A large percentage of today’s malware is Trojans.
A Trojan program pretends to do one thing, but actually does something different, usually without your knowledge. Popular examples are video codecs that some sites require to view online videos. When a Trojan codec is installed, it may also install spyware or other malicious software.
Another example is a malicious link that says “Cool Game.” When you download and install the game program, it turns out not to be a game at all, but a harmful Trojan that compromises your computer or erases the data on your hard drive.
Trojans are often distributed with pirated software applications and keygens that create illegal license codes for downloadable software. (See Backdoor Trojan)