A kilogram is the SI unit of mass and one of the 7 base units. It is the only SI unit with a prefix. Originally the gram was going to be the base unit, but due to a historical quirk regarding an artifact used to define the kilogram, the kilogram was chosen as the SI base unit for mass. The kilogram is currently the only base unit to rely on a physical artifact for its definition, which is more problematic than it sounds. For a fascinating essay on the metrology outlook on this problem please see NIST's website.