The four types of prefectures in Japan are to, do (or ), fu, and ken. The differences between the four types aren't much, however to is unique because it is reserved for Tokyo, a.k.a. Tokyo Metropolis. Prefectures are further separated into municipalities - shi, ku, cho (or chō), and son, which are cities, wards, towns, and villages respectively. The 23 wards of Tokyo are more like cities within Tokyo rather than only being named areas of the city, since each ward has an elected mayor and a certain degree of autonomy from the main Tokyo Metropolitan Government.
prefecture type  capital    population  area(km^2)   density  shi   ku   cho  son
Hokkaido   do    Sapporo     5,683,062   78,516.84     72.38   34   10   154   24
Aomori     ken   Aomori      1,475,728    9,606.33    155.84    8         34   25
Iwate      ken   Morioka     1,416,180   15,278.40     93.06   13         29   16
Miyagi     ken   Sendai      2,365,320    7,285.07    322.19   10    5    59    2
Akita      ken   Akita       1,189,279   11,612.11    103.13    9         50   10
Yamagata   ken   Yamagata    1,244,147    9,323.34    133.15   13         27    4
Fukushima  ken   Fukushima   2,126,935   13,782.48    154.79   10         52   28
Ibaraki    ken   Mito        2,985,676    6,095.58    491.44   22         45   17
Tochigi    ken   Utsunomiya  2,004,817    6,408.28    312.61   12         35    2
Gunma      ken   Maebashi    2,024,852    6,363.16    317.41   11         33   26
Saitama    ken   Saitama     6,938,006    3,797.25  1,816.64   41         40    9
Chiba      ken   Chiba       5,926,285    5,156.09  1,148.23   33    6    42    5
Tokyo      to    Shinjuku   12,064,101    2,187.05  5,404.01   26   23     5    8
Kanagawa   ken   Yokohama    8,489,974    2,415.41  3,488.34   19   25    17    1
Niigata    ken   Niigata     2,475,733   12,582.37    196.85   20         56   35
Toyama     ken   Toyama      1,120,851    4,247.22    264.74    9         18    8
Ishikawa   ken   Kanazawa    1,180,977    4,185.22    281.13    8         27    6
Fukui      ken   Fukui         828,944    4,188.75    197.68    7         22    6
Yamanashi  ken   Kofu          888,172    4,465.37    198.43    7         37   20
Nagano     ken   Nagano      2,215,168   13,585.22    162.27   17         36   67
Gifu       ken   Gifu        2,107,700   10,598.18    199.07   14         55   30
Shizuoka   ken   Shizuoka    3,767,393    7,779.46    483.85   21         49    4
Aichi      ken   Nagoya      7,043,300    5,155.84  1,345.08   31   16    47   10
Mie        ken   Tsu         1,857,339    5,776.40    321.81   13         47    9
Shiga      ken   Otsu        1,342,832    4,017.36    332.21    8         41    1
Kyoto      fu    Kyoto       2,644,391    4,612.94    555.67   12   11    31    1
Osaka      fu    Osaka       8,805,081    1,892.86  4,558.50   33   24    10    1
Hyogo      ken   Kobe        5,550,574    8,392.03    659.84   22    9    66     
Nara       ken   Nara        1,442,795    3,691.09    392.44   10         20   17
Wakayama   ken   Wakayama    1,069,912    4,725.55    230.16    7         36    7
Tottori    ken   Tottori       613,289    3,507.17    175.95    4         31    4
Shimane    ken   Matsue        761,503    6,707.29    113.63    8         41   10
Okayama    ken   Okayama     1,950,828    7,112.13    275.24   10         56   12
Hiroshima  ken   Hiroshima   2,878,915    8,476.95    338.82   13    8    67    6
Yamaguchi  ken   Yamaguchi   1,527,964    6,110.45    250.22   14         37    5
Tokushima  ken   Tokushima     824,108    4,145.10    200.54    4         38    8
Kagawa     ken   Takamatsu   1,022,890    1,875.88    550.81    6         33     
Ehime      ken   Matsuyama   1,493,092    5,676.22    265.82   12         44   14
Kochi      ken   Kochi         813,949    7,104.66    115.12    9         25   19
Fukuoka    ken   Fukuoka     5,015,699    4,971.01  1,001.65   24   14    65    8
Saga       ken   Saga          876,654    2,439.23    361.85    7         37    5
Nagasaki   ken   Nagasaki    1,516,523    4,092.44    373.22    8         70    1
Kumamoto   ken   Kumamoto    1,859,344    7,403.68    252.63   11         63   20
Oita       ken   Oita        1,221,140    6,337.97    194.77   11         36   11
Miyazaki   ken   Miyazaki    1,170,007    7,734.40    153.15    9         28    7
Kagoshima  ken   Kagoshima   1,786,194    9,186.71    194.11   14         73    9
Okinawa    ken   Naha        1,318,220    2,271.30    587.38   11         17   24
                   TOTAL = 126,925,843  377,873.06    335.90  675  151  1981  562

Little known fact: Osaka used to be the smallest area prefecture until the Kansai International Airport was built. Good luck Kagawa prefecture!

source: Japanese government, year 2000 census

This writeup of mine was originally found under "Prefectures of Japan."