The two measurements systems used in the typography.
Absolute measurements are easy to understand as they are measurements of fixed values. For example, as milimetre is precisely defined increment of a centimetre. Equally, both points and picas, the basic typographic measurements, have fixed values. All absolute measurements are expressed in finite terms that cannot be altered.
In typography, many values, such as character spacing are directly linked to typesize which means that they are defined by series ofrelative (rather than absolute) measurements. The basic building block for typographical characters, the em, is a relative measurement. Type set at 70pt has a 70pt em. Type set at 40pt has a 40pt em.