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)
Fullwidth Latin letters
Russian Cyrillic letters
Greek letters

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