In Typography, every character resides in an imaginary square called an Em Square. An Em Square is an imaginary square container that contains a coordinate system that directly correlates to the point size of the typeface (ie. If the font is set in 12pt then the em square will also be 12pt) The square is a direct descendant of the golden (or should I say lead) era of metal type. In metal type, each character sits on what is known as the body. All bodies must be the same height, from the capital M to the lowercase j in order to maintain straight lines. Looking at the metal type below, the individual letters all line up thanks to their body heights being the same.
More Imaginary Lines?!
We now have the understanding of where characters exist Starting with the baseline, the baseline is the invisible line where all characters sit. Think of it as the blue line on ruled paper. Next we have the x-height. the x-height is named after the character height of the lowercase x. All lowercase letters are as tall as the x-height (sans overshoots, but that for another time) Another obvious name is the cap-height, which is the height of the uppercase/capital letters in the given typeface. Finally we get to the ascender and the descender. The Ascender is the height of the lowercase letters that have a portion of their letter reaching above the cap-height such as: b,d,f,h,k, and l. The descender is the portion of the lowercase letters that extend below the baseline such as: g,j,p,q, and y.