Usually seen in upscale (read, expensive) homes. Encyclopedia Brittanica online uses this definition:
in architecture, a square or polygonal ornamental sunken panel used in a series as decoration for a ceiling or vault. The sunken panels were sometimes also called caissons, or lacunaria, and a coffered ceiling might be referred to as lacunar.
If the builder or homeowner wants to you to think they’re really fancy-schmancy, they extend the coffer into squares. This is usually seen in the office/den or the dining room.
Coffered ceilings have been around for a long time. An example from the 15th Century - a flat-panel coffered ceiling with gold overlay, in the Duomo, a medieval cathedral in Pisa, Italy next to the Leaning Tower.
picture credit to Wikipedia user Lofty
A modern example, from the Washington Metro station, Washington, D.C., USA, upper level tracks (circa 1968).
picture credit to Wikipedia user Kkmd
If you’re so inclined, Google Images has a great stock of different types of coffered ceilings.
