A pixel is a single point in a graphic image. It is the smallest item of information in an image.
A point. Today, the traditional point has been supplanted by the desktop publishing point (also called the PostScript point), which has been rounded to an even 72 points to the inch (1 point = 127⁄360 mm ≈ 0.353 mm). In either system, there are 12 points to the pica.
An em is a unit of measurement in the field of typography, equal to the point size of the current font. This unit is not defined in terms of any specific typeface, and thus is the same for all fonts at a given point size. So, 1 em in a 16 point typeface is 16 points.

Conversions of measure units, used on webpages: Point Pixels Em Percents. Press to mark table line.