A buoy is a floating device that can have various purposes, which determine whether the buoy is anchored (stationary) or allowed to drift: The word is derived from the Dutch 'boei'. In North American English it is pronounced as "Boo-ee", while in Commonwealth English, the pronunciation is virtually identical to "boy".

• sea mark - aids pilotage by marking a maritime channel, hazard and administrative area to allow boats and ships to navigate safely.
• lifebuoy - a life saving buoy designed to be thrown to a person in the water to provide buoyancy. Usually has a connecting line allowing the casualty to be pulled to the rescuer
• Submarine communication buoy - to be released in case of emergencies or for communication
• Communication buoy for a bottom pressure sensor, for tsunami detection.
• DAN buoy - has two meanings:
• a large maritime navigational aid providing a platform for light and radio beacons
• a lifebuoy with a flags used on yachts and smaller pleasure craft
• Sonobuoy - used by anti-submarine warfare aircraft to detect submarines by SONAR
• Surface Marker Buoy - taken on dives by SCUBA divers to mark their position underwater
• Decompression buoy - deployed by submerged SCUBA divers to mark their position underwater whilst doing decompression stops
• shot buoy - used to mark dive sites for the boat safety cover of SCUBA divers so that the divers can descend to the dive site more easily in conditions of low visibility or tidal currents and more safely do decompression stops on their ascent
• mooring buoy - to keep one end of a mooring cable or chain on the water's surface so that ships or boats can tie on to it
• tripping buoy - to keep one end of the tripping line on the water's surface so that a stuck anchor can more easily be freed
• weather buoy - equipped to measure weather parameters such as air temperature, barometric pressure, wind speed and direction and to report these data via satellite radio links to meteorological centres for use in forecasting. May be anchored or allowed to drift in the open ocean currents. Position is calculated by the satellite.
• profiling buoy - specialised model which adjusts its buoyancy so that it will sink at a controlled rate to 2,000 metres below the surface while measuring sea temperatures and salinity. Then after typically 10 days it returns to the surface and transmits its data via satellite before sinking again.
• ice marking buoy - buoys for marking ice holes in frozen lakes and rivers, so that snowmobiles do not drive over the holes.