In the Catholic Church, as well as in other Western forms of Christianity, the sanctuary refers to the area immediately around the altar. It is considered to be a place of the utmost divinity where the physical presence of God can be felt. For the most part, it is occupied by the priest and those participating in Mass, but it is often used by the congregation to pray or to take communion.
The word "sanctuary" means a sacred place and is often used as a word for the holiest places of worship, even within a house of worship itself. More antiquated services would have clear separation between the sanctuary and the rest of the area of prayer. However, in modern times, these are less separate, and often, "sanctuary" can simply refer to the room where a religious service takes place, such as a chapel.