(Adapted from "Inside your Church"). Originally, an outside, covered porch-like structure or an inside area separated from the nave (the "body" of the church) by a screen, but this word has come to mean "entry" or "foyer." Originally, penitents and Catechumens were confined to this area until their reconciliation with or initiation into the Church.
Nave. Referring to the "barque of Peter" and "Noah's Ark," the word "nave" is derived from the Latin word for ship, navis, and has come to mean the area where the parishioners sit or stand. Pews are a very late addition to the nave area, and, even today, parishioners stand during the liturgy in many Eastern Catholic and Orthodox Churches. In Gothic architecture, the nave had an aisle (or two) on both sides.
Chancel. The space around the altar at the liturgical east end of a church building. It may terminate in an apse.
Crossing. The place where the nave, chancel and transept intersect. This area is often domed.
Transept. The transverse arm of a cruciform church is called the transept. Because the liturgy is supposed to be celebrated ad orientem (facing East), the left side of the transept is called the North transept and the right side of the transept is called the South transept. This is so even if the actual orientation of the Church is other than with the Altar at the East side. Some churches have transepts at the West end of the church, too -- especially English Gothic churches.
Ambulatory. An ambulatory is basically a sort of walkway which can be either inside or outside of a structure. In Gothic architecture, ambulatories often had projecting chapels and were especially common around the apse. If an ambulatory is outdoors and is built such that one side is wall while the other has columns or arches, especially opening onto a courtyard, it is often called a cloister (the word "cloister" also refers to the area within a monastery to which some religious are confined).