One kilogram equals 1,000 grams. Both kilograms and grams are measures of mass in the metric system, so they are based on units of 10.
For most units of measurement in the metric system, the base unit is the one without a prefix, such as "meter." However, in mass, the base unit is the kilogram. The weight of a kilogram is based on 1 cubic decimeter of water, though it has been standardized through the international prototype of the kilogram (IPK) since 1889. The prototype, which is kept by the International Bureau of Weights and Measures, is made of plutonium and iridium.