Q. 1. What is a Church "Altar?"

A. 1. In a Catholic church, the Altar is the structure upon which the Eucharist is celebrated.

The Altar, centrally located in the sanctuary, is to be the focus of attention in the church. At the beginning of the Roman Rite of Mass, the priest first of all reverences the Altar with a kiss and only after that goes to the chair at which he presides over the Introductory Rites and the Liturgy of the Word. Except in Solemn Mass, a priest celebrating Tridentine Mass (use of the 1962 version of which is by the 7 July 2007 motu proprio Summorum Pontificum still authorized for use both privately and, under certain conditions, publicly) remains at the Altar the whole time after saying the Prayers at the Foot of the Altar.

The rite of dedication of a church includes that of the Altar of the church and celebration of Mass on that Altar is "the principal and the most ancient part of the whole rite" in accordance with the saying of the Fathers of the Church: "This Altar should be an object of awe: by nature it is stone, but it is made holy when it receives the body of Christ."

[Source: https://en.wikipedia.org/wiki/Altar_in_the_Catholic_Church]

Above, you will see two photos. On the left is the photo of a Traditional Altar made of stone on which the Tabernacle is attached.

The Traditional Altar sits against the wall at the front center of the Church. The priest celebrates the Holy Mass facing the Altar, with his back to the people.

On the right, you see a photo of a wooden Altar shaped like a table. Modern Altars can be made of stone, medal or wood, sitting on one to four legs, or being totally enclosed underneath.

The modern Altar sits in an open area in the front center of the Church. The priest stands behind the Altar, facing the people.