The Trojan Horse was a mythological device used during the Trojan War.
Homer, in The Odyssey,, described it as a large wooden horse on wheels. The Achaean League left this by the gates of Troy as a supposed tribute. Secretly however, the Horse was made hollow, and filled with Greek/Achaean soldiers, under the leadership of Odysseus. This attack force came out at night and opened the city gates to let in the Achaean army and attack the city.
Some military theorists suggest instead that the Trojan Horse was a straightforward battering ram that the Achaeans used in a night action.
In modern computing, the term Trojan Horse has come to refer to a computer program which appears to have one function, but in reality has a completely different one that's harmful.