Shift-JIS

Shift-JIS encoding is natively used in the Japanese edition of Windows 9x and MS-DOS.  Letters supported are:

Kanji (fullwidth)
Hiragana (fullwidth)
Katakana (halfwidth and fullwidth)
ASCII
Fullwidth Latin letters
Russian Cyrillic letters
Greek letters
Symbols

However, Latin letters with Diacritical marks (e.g. ä, ò) are not supported.

Shift-JIS characters are variable width; either 16 bits or 8 bits.  This differs from UTF-8, which can have characters longer than 2 bytes. Halfwidth Katakana and ASCII fonts are represented in 8 bit characters.  ASCII characters directly map onto Shift-JIS, save the backslash (\), which is used for the Japanese Yen currency sign in Shift-JIS.  This node will display fine if displayed as Shift-JIS.  Extended ASCII for Western European languages will look like an odd mixture of ASCII and halfwidth Katakana, however. 

Shift-JIS doesn't display ASCII art properly because all the backslashes become Yen signs. Japanese characters are frequently displayed as fullwidth monospace to fit into an invisible grid of squares.  To keep consistency, fullwidth English letters were invented so English letters, too, can fit nicely into squares.  One reason for the popularity of monospace Japanese font is due to the fact that Japanese is sometimes written vertically.  In which case, it is essential that fonts have a fixed width for aesthetics.

See also: Japanese Character Encoding Formats, Please use plain text

A note on the history of Shift-JIS, as opposed to the older JIS standard:

The first Japanese character encoding used on computers was JIS X 0201, which replaced the ASCII characters over 127 with tiny katakana characters called hankaku kana. Early Japanese computers could only print Roman letters and these ugly half-width katakana, which are still seen today on receipts, invoices, and the like. When JIS was upgraded to JIS X 0208, the half-width katakana were replaced with other characters.

Microsoft decided to bring back the half-width katakana to make their version of JIS compatible with older JIS X 0201 files, so they shifted the entire set over 127 places: hence the name "Shift-JIS." Shift-JIS is identical to regular JIS, except that all the characters after the standard ASCII are moved forward 127 values. (BTW, this is why accented European characters tend to farg up on Japanese computers.)

For more info on Japanese character encoding, I strongly recommend Jim Breen's guide at http://www.csse.monash.edu.au/~jwb/coding_inf.html.