From the Wikipedia page 
The Prefectures of Japan (都道府県 Todōfuken?) consist of 47 prefectures. They form the first level of jurisdiction and administrative division of Japan. They consist of 43 prefectures (県 ken?) proper, two urban prefectures (府 fu?, Osaka and Kyoto), one "circuit" or "territory" (道 dō?, Hokkaido) and one "metropolis" (都 to?, Tokyo). The Meiji Fuhanken sanchisei administration created the first prefectures to replace the provinces of Japan in 1868.
Each prefecture's chief executive is a directly-elected governor (知事 chiji?). Ordinances and budgets are enacted by a unicameral assembly (議会 gikai?) whose members are elected for four-year terms.
Under the current Local Autonomy Law, each prefecture is subdivided into cities (市 shi?) and districts (郡 gun?) and each district into towns (町 chō/machi?) and villages (村 son/mura?). For example, Hokkaido has 14 subprefectures that act as branch offices (支庁 shichō?) of the prefecture. Some other prefectures also have branch offices that carry out prefectural administrative functions outside the capital. Tokyo, the capital of Japan, is a merged city-prefecture; a metropolis, it has features of both cities and prefectures.