The Kilogram is the fundamental unit of mass in the International System of Units. Previously, when it was created, the kilogram was defined as being equivalent to the mass of one liter of demineralized water at a temperature of fifteen degrees Celsius. Later, this equivalence was abandoned, when it was realized, that the mass of the same amount of water varied according to its purity. Since most of the objects used by man in his day-to-day life are relatively larger than the gram, and since water does not have the same density in all his samples, a standard was required at the commercial level that could be reproduced and maintains its stability. Thus, the mass standard was defined as exactly equal to the mass of a small polished cylinder, fused in 1879 of platinum and iridium, maintained in France, a thousand times greater than the gram, the kilogram.
The Gram is a unit of mass and is one-thousandth of the standard kilogram. Although the kilogram is the fundamental unit of mass, in practice we use gram as the main unit. This is because, the multiples and submultiples of the mass units, are defined from the gram, see the case of hectogram, decagram, decigram, centigram, etc. Did you know that a cap of a pen weighs about a gram and that this is also the approximate weight of a paper money?