Short answers, pulled from the story.
The Trojan Horse was a massive wooden structure constructed by Odysseus on orders from the Greeks. It contained hidden warriors who emerged at night to open the gates for the main army and end the ten-year siege of Troy.
Virgil wrote his detailed account of the event in 19 BC as part of the Aeneid epic poem. Earlier texts like the Little Iliad and Sack of Troy contained fuller versions but survive only in fragments today.
Thirty Achaean warriors hid inside the belly of the wooden construct according to early traditions. Late tradition standardized the count at forty men while other sources claim different numbers such as fifty or twenty-three.
Euripides wrote that Phocian Epeus framed a horse to bear an armed host within its womb in his play Trojan Women of 415 BC. Pausanias also stated that Epeius created a contrivance to breach Trojan walls during the 2nd century AD.
A Boeotian fibula dating from about 700 BC shows one of the earliest depictions of the device. The Mykonos vase spans the period between 750 and 650 BC and displays faces of hidden warriors on its side.