Relative Units:
em - the 'font-size' of the relevant font
ex - the 'x-height' of the relevant font
px - pixels, relative to the viewing device
Absolute Units:
in - inches
cm - centimeters
mm - millimeters
pt - point - 1/72 of an inch
pc - pica - 12 points
Examples:
font-size: 1.25em; border-top-width: 0.5cm;