Buoys, in the oceanographic context, are platforms whose defining characteristic is that they float at a predetermined depth in the ocean. Most often BUOYS float on the surface of the ocean.
... Typically, BUOYS are divided into two categories: drifting and moored buoys.
Moored BUOYS are floats fixed in water to mark a location, warn of
danger, or indicate a navigational channel. They can also take
scientific measurements of the water including temperature and
water quality measurements.
Drifting BUOYS are exclusively scientific platforms that drift along the surface of the ocean following the ocean currents and/or the surface winds depending on their configuration. Along the trajectory, they collect oceanographic or meteorological data. 
