The hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.  A vessel floats because water exerts a buoyant force on the hull. This force exactly equals the weight of the water that is displaced by the hull. In order to float, the vessel must weigh no more than the water it displaces - this combined with total surface area is what dictates draft 
Categorization of hulls
A displacement hull is one that achieves all of it's buoyancy by displacing a volume of water equal in weight to the boat and its load, whether at rest OR underway. The hull is supported exclusively or predominantly by buoyancy. Vessels that have this type of hull travel through the water at a limited rate that is defined by the waterline length. They are often, though not always, heavier than planing types. All cruising sailboats and large, low-speed powerboats (such as trawlers) take advantage of the low power demands of this type of hull. Little driving force is needed to move one of these boats until hull speed is reached. After this, no reasonable amount of increased power results in any efficient increase in speed. A close approximation of any vessel's hull speed in knots can be found by multiplying the square root of its length at the waterline (LWL) in feed by the constant 1.3. For example a boat with a LWL of 36 feet has a theoretical hull speed of 7.8 knots: √ 36 x 1.3 = 6 x 1.3 = 7.8. The need for an easily driven hull in a sailboat comes from the limit of force that can be generated from safe winds. In a powerboat, this type of hull allows long-range cruising with low fuel consumption. Trawlers and similar can easily have a range of several hundreds of nautical miles (and probably more). Another advantage of these hulls, is the ability to carry heavy loads with little penalty in performance. 
Displacement hulls are almost always round bottomed.
A planing hull is one that achieves a major part of its underway-load-carrying ability by the dynamic action of its underside with the surface of the water over which it is rapidly traveling. At rest, a planing hull reverts to displacement buoyancy. The planing hull form is configured to develop positive dynamic pressure so that its draft decreases with increasing speed. The dynamic lift reduces the wetted surface and therefore also the drag. They are sometimes flat-bottomed, sometimes V-bottomed and more rarely, round-bilged. The most common form is to have at least one chine, which makes for more efficient planing, aid in speed and direction, and can throw spray down. Planing hulls are more efficient at higher speeds, although they still require more energy to achieve these speeds. An effective planing hull must be as light as possible with flat surfaces that are consistent with good sea keeping. Nearly all modern motorboats have planing hulls. Sail boats that plane must also sail efficiently in displacement mode in light winds.
Semi-displacement, or semi-planing
Tthe hull form is capable of developing a moderate amount of dynamic lift; however, most of the vessel's weight is still supported through buoyancy 
- , Wikipedia
- Chapman, Charles F. (2013). Chapman Piloting & Seamanship (67th ed.). pp. 19.