If you use a comma in maths mode, you get a small space after it; this
space is inappropriate if the comma is being used as a decimal
separator. An easy solution to this problem, in maths
mode, is to type 3{,}14 instead of typing
3,14. However, if your language's
typographic rules require the comma as a decimal separator, such usage
can rapidly become extremely tiresome. There are two packages that
can help relieve the tedium: icomma and ziffer.
Icomma ensures that there will be no extra space after a
comma, unless you type a space after it (as in f(x, y), for
instance), in which case the usual small space after the comma
appears. Ziffer is specifically targeted at the needs of
those typesetting German, but covers the present need, as well as
providing the double-minus sign used in German (and other languages)
for the empty 'cents' part of an amount of currency.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=dec_comma