The letter-spacing property controls the
amount of spacing in between letters. This property can be set in any one of
the available CSS length units: px, %, em, in, cm, mm, pt, pc, and ex.
One important note to mention about this property is that any amount you
specify will be in addition to the default amount
of spacing between characters. Negative lengths are also permitted.
Note: Character spacing may also be
affected by justification (see the text-align property).
Tip:
This property is optional. If you would like to include it in your
generated CSS code simply check the checkbox by it's name (to enable it)
and then enter in a value and choose a CSS length unit. For example, I
may type in "1" for it's value and "px" for it's length unit, making it's
total value "1px". That means the spacing in between letters will be
"1px" (the amount I specified) plus whatever their normal or default
spacing was. If you would like to decrease the amount of spacing in
between letters just use a negative amount. For an explanation of all
the different CSS length units, consult the section of the manual
entitled CSS Length Units.
Read
an official description of the letter-spacing property from the W3C