Letter-Spacing

Increase or decrease the space between characters in text.

Syntax
      letter-spacing: length | normal | inherit ;

Length - The letter spacing in px, pt, em, etc.
normal - The default letter spacing.
inherit - Inherit from the parent element.

A negative length will bring the letters closer together than normal (compressed text).

Using font-relative values (em or rem) is recommended, so that the letter-spacing will increase or decrease by an appropriate amount if the font-size is changed.
The length value is added to (or subtracted from) the browsers default spacing (which is based on the font metrics.)

It is rarely if ever a good idea to letter-space lower case letters.

Most current browsers now support subpixel values (anything that computes to less than 1px.)

The default value is normal

Examples:
h1 { letter-spacing: 2px; }
.emw3class { letter-spacing: 0.5em; }
#emw3id { letter-spacing: -1pt; }

Try it:

(CSS 1) Browser Support: All major browsers (the degree of spacing does vary).

“Everything is worth what its purchaser will pay for it” ~ Publilius Syrus

Related:

word-spacing - Increase or decrease the space between words.
font-size - Font size of text.
font-variant - Whether or not text should be displayed in a small-caps font.