A horrible system of units
that plagues our good society and causes engineering students (American ones, anyway) to weep in terror and frustration.
Basically, it defines a system of units; most of these units are common (i.e. the second for time), so those of length, force, and mass are the most important to define.
Length is familiar--feet or inches or miles.
Force is also familiar--commonly pounds. Often written as lbf (pounds force) to avoid the confusion of the next unit...
Mass is where we get into trouble. Most people have no idea what mass is, and don't care--they don't need to, either. Basically, mass is an object's resistance to linear acceleration--how much 'stuff' it holds. Weight is actually a force, and related to mass by f=ma.
In this system, mass is measured in pounds mass, or lbm, which most people have never heard of. Basically, this system came about back before Newton's Second Law of Motion, so people didn't understand the relation. They wanted one pound mass to weigh one pound. So on earth (where gravity is 32.2 ft/sec^2), 1 lbf = 1 lbm * 32.2 ft/sec^2. As you can see, the units don't add equate (lbf = ft*lbm/sec^2), so we must divide the mass by the horror that is gc to make it work.
British Gravitational is easier to use, and SI is much easier.