A coffered ceiling is created with coffered panels or "coffers." Coffers are sunken panels attached to a suspended (drop) grid to create a new ceiling with depth and architectural interest. Each lightweight rigid PVC (plastic) coffered panel is 24" x 24", available in either deep or shallow styles, and comes in white or black. Deep coffers add visual weight to a room, while shallow coffers create a lighter feel.
You can install a coffered ceiling in a new or existing 24" x 24" suspended ceiling grid. Since no room is perfectly square, you'll also need to create a perimeter border, using either a tegular edge mineral fiber drop ceiling panel or drywall. A drywall border is the preferred option since it will provide a high-end, custom look.