Atoll: A ring-shaped coral reef including a coral rim that encircles a lagoon there may be coral islands on the rim. It usually exists on top of an extinct volcanoes.
Guoyts: are flat-topped seamounts located in several hundred to several thousand meters underwater. They usually are former volcanoes that are in many cases capped by drowned atoll reefs and pelagic sediments.
The difference is that atolls are grown towards the surface while guyot remains under the sea.
Atoll reefs occur in all oceans; they are most common in the Indo-Pacific realm.