In the context of computing, a Trojan horse is malicious software which is used to hack into a system or computer by misleading users of its true purpose – which is why its name takes after the wooden horse that helped Greek soldiers invade Troy in the Ancient Greek story.
Trojans are commonly spread via social engineering – like a user clicking on an email attachemt which looks legitimate – or by drive-by download. Trojans do not inject themselves into other files or propagate themselves, unlike viruses and worms.
Trojans have many uses. They can be used for destructive purposes, for example used to crash the computer or corrupt data, or they can be used for data theft, money theft or ransom, or used to infect a machine to be used as part of a botnet.
Trojans can even be used as part of industrial espionage and surveillance, with certain Trojans allowing keystroke logging, remote control of the system and accessing the user’s screen.