A headland is an area of land that is surrounded by water on three sides. Very often, the land areas are called capes. A bay is an area of water. It is surrounded by land on three sides. The water areas are also called gulfs.
A bay is a large body of water in the land next to a sea or lake between two headlands. The waves coming to the shore in a bay are usually constructive waves, and because of this, many of them have a beach. A bay may be metres across, or it could be hundreds of kilometres across. A bay often contains beaches.
Bays form where weak rocks, such as sands and clays, are eroded, leaving bands of stronger rocks, such as chalk, limestone, or granite, forming a headland, or peninsula. Headlands and bays are formed when there are parallel sections of softer and harder rock perpendicular to the coast. The sea erodes the softer rock faster than the harder rock, forming a bay. The harder rock that is left protruding into the sea is the headland.
They also can be made by people mining off the coast.
Hanauma bay, on the island of Hawaii