The adjustment of the space between letters to create a better aesthetic is an old tradition in graphic design. There are two terms that describe how the change in the space is adjusted: kerning and tracking. Kerning is a design term used to describe the changing of the space between a pair of letters to create a better visual effect. An example of kerning is adjusting just the space between an uppercase T and a lowercase o. Tracking involves more than a pair of letters; it adjusts the space between letters over a large amount of text.
O'Reilly Network - Manipulating Space with CSS
CSS Example:
p
{
letter-spacing: 12px
}
p
{
letter-spacing: -0.5px
}
This has a letter-spacing of 5px between letters
This has letter-spacing of -.1em between letters