px: pixels
em: an em is the height of the element's font as rendered on a given display device. so if you set a font-size for the paragraph (P) element to be 14px, then assign the large class a font-size of 3em, then the class font size becomes 42 pixels (3 x 14).
ex: the height of a lowercase "x"
in: inches
cm: centimeters
mm: millimeters
pi: picas
pt: points (1 pt = 1/72 inch)