A hull is the body of a ship or boat. It is a central concept in floating vessels as it provides the buoyancy that keeps the vessel from sinking.

General features
Almost all watercraft, from small boats to the largest ships, have a general form that is necessary for  stability and efficient propulsion, which includes:
 horizontal cross-sections that have narrow, usually pointed, fronts (at the bow), 
 smooth widening from the bow until roughly the middle of the length (midships), and often narrowing smoothly but usually significantly to the extreme end (the stern).

 A keel may be fitted on a hull to increase the transverse stability (if filled with a heavy weight), directional stability or to create lift as in a sail boat . 
 Control devices such as a rudder, trim tabs or stabilizing fins may be fitted.

How does it float? 
Archimedes, a Greek doctor, inventor, and mathematician, made the science of "hydrostatics", the study of liquids and pressure. The Archimedes principle says that the force holding up an object that is inside a liquid in part or totally. So, a ship actually does sink, until it moves an amount of water that is the same weight of the weight of the ship. The weight must be over a wide area to work. This explains why ships and boats (including submarines) float. An object's weight pulls down in the direction of gravity, but the water's buoyant (floating) force pushes it up. Because these forces counteract each other, the object seems to lose weight. NASA uses this principle to prepare their astronauts for the weightlessness of space, so they had the astronauts work on parts of the space ship inside water tanks 25 feet (7.6 m) deep.

Terms
Bow is the frontmost part of the hull

Stern is the rear-most part of the hull

Portside is the left side of the boat when facing the Bow

Starboard is the right side of the boat when facing the Bow

Waterline is an imaginary line circumscribing the hull that matches the surface of the water when the hull is not moving.

Midships is the midpoint of the LWL (see below).  
It is half-way from the forwardmost point on the waterline to the rear-most point on the waterline.

Baseline an imaginary reference line used to measure vertical distances from.  It is usually located at the bottom of the hull.

Metrics

Hull forms are defined as follows: 
 Block Measures that define the principle dimensions.  They are:
 Length Overall (LOA) is the extreme length from one end to the other (see also o/a)
 Length on the Waterline (LWL) is the length from the forwardmost point of the waterline measured  in profile to the stern-most point of the waterline. (see also w/l)
 Beam or breadth (B) is the width of the hull.  (ex: BWL is the maximum beam at the waterline)
 Depth (D) is the vertical distance from the top of the keel to the underside of the upper deck at the side.
 draft (d) or (T) is the vertical distance from the bottom of the hull to the waterline.
 Freeboard (FB) is depth plus the height of the keel structure minus draft.
Form Derivatives that are calculated from the shape and the Block Measures.  They are:
 Volume (V) is the volume of water displaced by the hull.
 Displacement (Δ) is the weight of water equivalent to the immersed volume of the hull.
 Longitudinal Centre of Buoyancy (LCB) is the longitudinal distance from a point of reference (often Midships) to the centre of the displaced volume of water when the hull is not moving.  Note that the Longitudinal Centre of Gravity or centre of the weight of the vessel must align with the LCB when the hull is in equilibrium.
 Vertical Centre of Buoyancy (VCB) is the vertical distance from a point of reference (often the Baseline) to the centre of the displaced volume of water when the hull is not moving.
 Longitudinal Centre of Floatation (LCF) is the longitudinal distance from a point of reference (often Midships) to the centre of the area of waterplane when the hull is not moving.

Related pages
ship
submarine

References

Other websites 
Nautical terms for the parts of sailing vessels
Description of hydrostatics and coefficients

Ship building