Points
Most people are quite familiar with the "point": "Your essays must be in 12pt font." A point is an absolute measurement that has been contested over the years, but now is set by standards to be 1/72 of an inch. This means that it is the same no matter the size of the font, and is why we speak of font sizes in terms of points, rather than other measurements that we will come to shortly.
picas!
A pica, on the other hand, is equal to 12 points, or 1/6th of an inch. We won't be working much with picas on the web, but they come up if you start using page layout software.
ems
In contrast to the point and the pica, an em is a relative measurement: one em is equal to the point size of the font. So, if we have a 72 point font (which, remember, is going to be 1 inch wide by definition), then an em is 1 inch. If, on the other hand, we have a 12 point font, then an em is going to be 1/6th of an inch (12 pt divided by 72 points per inch). This relative nature of things comes in very handy when we start working on the web.
Note: most people think that an em is equal to the width of the capital "M" in a particular typeface. While this might have been the case historically, it is more often than not the case today.
An "em-dash" is, as you would expect, a dash that is exactly one em wide. This is often how you would set off a phrase that needs some emphasis—as we are doing here.