A point (unit pt) is used for measuring text size on paper and on screen. On paper, a point is about 0.35mm, and on screen a pt is equal to 1.3333333... pixels1. 12 points equal 1 pica.
The point was adopted as a standard measure in typography in America in 1878, and in England in 1898. Prior to this each foundry had adopted their own system of measure. In Europe however, the point was set at .376mm. To minimise confusion, this is refered to as the Didot point.
http://www.dtp-aus.com/typo/pointsys.shtml was very helpful
As was http://style.cleverchimp.com/
1. At 96dpi (Windows Standard). On Macs a pixel is equal to a point. I'm sure there's others too.