In horizontal spacing, an em space measures the width of printed matter. All the other spaces are based on the em space.

UNIT	DEFINITION	UNICODE
em space	equal to typesize, i.e. an em space in 12-point text is 12 points wide	2003
en space	half of an em space	2002
thick space	a third of an em space	2004
medium space/mid space/word space	a fourth of an em space	2005
thin space	a fifth or a sixth of an em space	2009
hair space	thinner than a thin space (sometimes by half)	200A

To produce the space, type the code on the numeric keypad, then press Alt together with x.