A ward (区 ku?, Nihongo) is a subdivision of the cities of Japan that are large enough to have been designated by government ordinance. Wards are used to subdivide each city designated by government ordinance ("designated city"). The 23 special wards have a quasi-municipal status, and are not the same as other entities referred to as ku, although their predecessors were.
Wards are local entities directly controlled by the municipal government. They handle administrative functions such as koseki registration, health insurance, and property taxation. Many wards have affiliated residents' organizations for a number of tasks, although these do not have any legal authority.
More information on the Wikipedia page .