Basically: line-height sets the height of a line of text.
If you make the value larger than the font-size (FontSize) then it will look like the lines are moving further away. Set it smaller, and the lines will start to overlap.
For a technical definition, read the [W3C on line-height].
An example from the specs:
div { line-height: 1.2; font-size: 10pt } /* number */
div { line-height: 1.2em; font-size: 10pt } /* length */
div { line-height: 120%; font-size: 10pt } /* percentage */
Each example will have the same immediate effect, but they differ in inheritance. Both em and % inherit as computed values, while a unitless number inherits directly. Felix has an [example] that shows the significance of the difference.
This can be done using the ShorthandProperty font.
Example:
font: 12px/1.4 sans-serif;
This will set font-size to 12px, line-height to 1.4 and font-family to sans-serif.
font-size always comes first, line-height comes second:
font-size/line-height
NB: The font-family part is neccessary.
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