A turret is a small tower that is taller than the rest of the building. The long-haired fairy tale heroine Rapunzel had to stay in a turret.
The classic turret is perched at the very top of a stone castle and is often small and rounded, with tiny windows. Another meaning of turret is the structure on a tank or fort that's heavily protected with armor and contains a gun. Often this kind of turret is rounded and can rotate, in order to fire in different directions.