Definition
A Trojan horse is malware that disguises itself as a normal, useful program so you install it yourself. Once it's in, it quietly takes control of your computer or steals information from inside it.
You Opened It Thinking It Was a Gift
In Greek myth, the Greek army left a giant wooden horse outside the gates of Troy as a gift and sailed away. The people of Troy thought it was a trophy of victory. They happily dragged it inside the city walls. But when night fell and everyone was asleep, enemy soldiers hiding inside the horse climbed out and threw the city gates wide open.
A Trojan horse in the computer world works in exactly the same way. It is carefully disguised as useful software, like a free installer for a popular paid game or a document editor. Security systems and users judge by appearances, so people trust it as a useful program and click install themselves.
The moment you click install, it looks like the normal program you were promised is running. But behind the scenes, out of sight, the hidden malicious code starts running quietly and begins taking over your computer.
How Is It Different from Viruses and Worms?
People often lump every program that causes computer trouble together and call it a virus. But from a security point of view, the types are strictly divided by how they infect. A program that attaches itself to other normal files and spreads by copying itself is a computer virus. A program that crawls from one computer to another on its own through a network is called a worm.
A Trojan horse, by comparison, has no ability at all to attach itself to other files or copy itself and spread. It mostly gets into a computer when a user is fooled and clicks an executable file on their own. Sometimes another piece of malware that is already inside quietly downloads and installs it.
Since it can't multiply by itself, a Trojan horse focuses on tricks that play on a person's curiosity or impatience. Instead of an attacker forcing their way in from outside, it gets the victim to open the door with their own hands. It's a psychological attack technique.
A Little More Precisely: The Start of Backdoors and Botnets
To be a little more precise, a Trojan horse usually doesn't finish its damage on its own. It acts as the first gate to a bigger security breach. Once settled in the system, it quietly builds a backdoor, a secret passage that lets an outside attacker come and go whenever they like.
Once this secret passage is open, a remote attacker can control the victim's computer however they want. They might plant a keylogger that captures the passwords you type, or secretly watch your screen. They might also link huge numbers of infected computers together like zombies and use them as tools for a DDoS (distributed denial-of-service) attack that knocks a website offline.
Because a Trojan horse gets in with the user's permission, it's not enough to rely only on the watch of security software. It's important to build the habit of strictly avoiding email attachments from unclear sources and downloads from unofficial channels.
Common misconceptions
A Trojan horse multiplies on its own and spreads to other files or computers.
It has no ability to copy itself or spread over a network. In most cases, it starts working only after a user is fooled into downloading and running the file.
Where you meet it
It sneaks in hidden inside pirated files that promise to let you use paid software or games for free.
It gets quietly installed when you open an email attachment disguised as a package-tracking notice or a bill.
In one sentence
Malware that disguises itself as a normal program so you install it yourself, then builds a secret passage into your computer to help hackers.