Versus populum (Latin for "towards the people") is the liturgical orientation in which the priest celebrates Mass facing the people. The opposite orientation, whereby the priest faces in the same direction as the people, is often called ad orientem ("towards the east"), even if the priest is not in fact facing the east.
From the middle of the seventeenth century, almost all new Latin-rite altars were built against a wall or backed by a reredos, with a tabernacle placed on the altar or inserted into the reredos. This meant that the priest turned to the people, putting his back to the altar, only for a few short moments at Mass. However, the Tridentine Missal itself speaks of celebrating versus populum, and gives corresponding instructions for the priest when performing actions that in the other orientation involved turning around in order to face the people.
Read more about Versus Populum: Earliest Churches in Rome, Other Pre-twentieth-century Churches in Rome, 1970 Roman Missal, Tabernacle On The Altar, Criticisms