To understand how type works, you must know how it is measured. Basically, typefaces can be measured in two ways: height and width.
In earlier times when type was molded out of metal, it was sold in discrete sizes that were measured in points. Today's digital types can be enlarged or reduced by simply selecting, or specifying, a point size.
Originally, the term point size referred to the height of the metal body that held the characters. This was slightly larger than the distance from the highest to the lowest feature in the design.
A traditional point is approximately 1/72 of an inch or .01384 inch. With the advent of desktop publishing, the point became exactly 1/72 of an inch. 12 points = 1 pica, and 6 picas = 1 inch.
This method of measuring is still used for digital type. Often, because some faces have very long ascenders and descenders, these typefaces look smaller than others when both are printed at the same point size. This incongruity is illustrated below.
In addition to height, a typeface is commonly measured by its width. The width of a typeface is often expressed in characters per pica, that is, the average number of characters that will fit within a pica. Characters-per-pica information is used for copyfitting to estimate whether text set in a specific typeface will fit into an allotted space or, conversely, to estimate how much space a given piece of text will occupy. This information can also be used to compare the relative widths of different typefaces.