Sacristy.
It is a room of preparation and (ideally) silent prayer. This is the room where the priest or sacristan (usually a lay person put in charge of maintenance of the sacristy and liturgical items) prepares the vessels and linens for Mass, and where the priest vests (puts on vestments) for Mass. In many Churches there are two or more sacristies, consisting of the main sacristy where liturgical vestments and vessels are stored and where the priest vests for Mass, an altar server sacristy where altar servers vest in cassock and surplice, or other specified robe, and sometimes even a third sacristy for work (cleaning candles, vestments, etc.).
it is called the ambulatory.
A Reredos.
Tiaria said its called the pulpit
It would still be called a chapel unless if it is just an altar, then it would just be called an altar. A chapel is a holy room reserved for prayer and practising of faith only.
From the way it sounds, it seems you are referring to the Altar.
It is called reredos. It usually has iconography or painting depicting an episode of bible.
An altar is actually called "the altar", although if there are many altars in a Church, they may be called by a specific name, like the main altar, the high altar, the side altar, the Blessed Virgin Mary altar, the altar of repose, the Blessed Sacrament altar, the St. Joseph altar, etc.
William and Kate got married in Westminster Abbey. William waited at the altar, and Kate got walked up to the altar by her father. After a couple of readings, songs, and talking, they went in a room behind the altar to sign papers. They had put the rings on and said the vows before they went behind. When they came back out, they were officially married! :)
in the tabernacle behind the altar.
Consecrated Hosts are stored in a locked Tabernacle, not on the altar. The Tabernacle may be located immediately behind the main altar or on its own pedestal in the Church, non on the altar.
Behind the door lies a hidden room called the underwater room in club penguin. Many girl penguins like to play "mermaids" in this room. There is also a pin in here called "moss key'' pin.
Apse