A Catholic altar is primarily used for the Holy Sacrifice of the Mass, where we believe a priest consecrates bread and wine into the substance of the Body and Blood of Jesus Christ. Catholic altars are traditionally made of stone, often marble, or wood. Before Vatican II, regardless of its material, a Catholic altar had to have an altar stone containing the relics of a Catholic martyr, thus making an altar a true altar. This altar stone is usually a flat square tablet, several inches by several inches with five crosses cut into it in an "X" pattern along its top surface; this stone is inset in the front top surface of the altar where the priest would reverence it during Holy Mass with several ceremonial kisses. The altar stone is usually difficult to spot as most altars are covered with linens during ceremonies and covers when not in use. If an altar stone is removed, the altar is desecrated and must be reconsecrated. Tabernacles, the little box-like compartments once found on most altars, were usually made of the same substance and style of the altar, though, according to Canon law, they had to be anchored to the altar so as not able to be moved. In the modern Church, tabernacles are rarely installed or have been allowed to remain on the altar and altar stones are all but discontinued save in traditional or pious channels.
In a nutshell, any flat surface can serve as an altar. A Greek corporal - a portable "altar stone" with relics sewn into it - can then be used by the priest. Mass can then be said on anything from a card table in a hotel to an ammunition crate in a war zone, as has been done by missionaries and military chaplains. In a case of emergency, Mass can be said without an altar stone almost anywhere, as the case of Cardinal Mindszenty who said Mass on his own chest while in prison. (sweet!)