A point is a very small unit of measurement used by those in the printing industry. Printers measure using a pica ruler and often talk of measurements in terms of picas and points, which are units that are part of the English measurement system. There are six picas in an inch. Each pica contains 12 points. That means that there are 72 points in an inch.
When type was set using individual blocks of lead for each letter and old fashioned printing presses, each block of the same size type was the same height. This height left room for the letter and also a little breathing room between lines, so the line of text on top didn't touch the line below. This height is the size of the font, and it's measured in points. But you have to keep in mind the point size is the height of the entire block of lead, not the height of the actual letter.
Even letters that are the same point size may be different heights. One reason is that the designers of two different fonts may design them differently, so a capital "E" in Garamond may not be the same height as a capital "E" in Helvetica, even if the fonts are the same size. And a capital "A" will be taller than the same point size lower-case "a," in any font.
Measuring font point size is not an exact science. Printers generally compare the size of a square capital letter, such as an "E," to a printed sample. This is done with a clear ruler that has various size letters printed on it, so you can lay it on the sample and see through it to compare the size. Depending on the font, the size may have to be adjusted after a best guess is made and another printed sample is produced.
In a computer program such as Microsoft Word, Microsoft Publisher or Adobe Pagemaker, the point size is the number you specify for the font, which may be called point size, type size or font size. Generally, text is set in 10 or 12 points, and headlines start at 14 and go up to 72 or more. Bibles, phone books and text in footnotes may have smaller text sizes, such as 7, 8 or 9 points.