Displacement is the truest measure of just how much boat you're dealing with. In a nutshell, it's the actual volume or weight of the vessel.
Of course, there are more than a few ways to state displacement, but only two- the volume displacement and the displacement mass, both at the design waterline- are fair points of comparison between different boats. The rest, although important and helpful when used properly, can be deceptive when seen alone. Always ask which weight is really being discussed, and what's included in it!
Units: Volume measure (cubic metres or cubic feet) for volume displacement, or mass (kilograms/tonnes or pounds/long tons) for the others.
Volume displacement ($\nabla$ or D)
The total volume of water displaced by the boat when she's sitting on her design waterline. Or, equivalently, the below-waterline volume of the hull and appendages.
Displacement at design load, or displacement mass (mLDC)
The weight of the boat when she's loaded to her design waterline. By Archimedes' principle, displacement mass is equal to displacement volume multiplied by the density of the water (nominally 1000 kg/m3 or 62.4 lb/ft3 for fresh water, 1025 kg/m3 or 64 lb/ft3 for seawater). This is the figure that should be used for all performance ratios and comparisons.
Displacement light-ship, or empty boat displacement mass, or dry weight (mLCC)
The weight of the boat when she leaves the build shed, before there's any personal gear brought aboard and before any fuel or water are put in the tanks.
Although it's important to determine mLCC when designing and building a boat, it'll never be sailed this light. Performance ratios calculated using the dry weight are worse than useless; nevertheless, some marketing materials list only this weight and numbers based on it in an attempt to trick the buyer into thinking the boat is faster and sportier than it really is.
Light load displacement mass (mMOC)
The weight of the boat with (usually) half-full tanks, a standard complement of essential gear, and a small crew. This is the absolute lightest weight the boat is ever likely to be sailed at, and performance ratios based on mMOC can be deceptively racy.