The light year is used for expressing the distances between stars. As you can see, if you only use kilometers or meters, the numbers would get out of hand very fast. 1 kilogram is the mass of 1 liter of pure water. We will be most often use masses in kilograms. Mass is the amount of matter in an object.
Weight is the force of gravity on an object.
Of the two, Mass is more fundamental. In everyday usage, we often erroneously use kilograms as a unit of weight. 2001 January 2 Copyright Richard W. Pogge, All Rights Reserved.