Leading is simply the spacing between lines, so called because in the days of handset type thin strips of lead alloy were inserted to provide such space. Letter spacing and word spacing, as the names imply, are the distances between letters or words. All three types of spacing can be controlled in many computer applications, often very precisely. Typefaces are divided in several way. First, there are monospaced and proportionally-spaced faces.
Typefaces are also divided into two categories in another way.
They either have finishing strokes, called serifs, like Times Roman, or are plain, without serifs, like Arial and Helvetica. Serifs are not just embellishments in type. They play major roles in the readability, appearance and style of a face. The more frequently used small letters occupied the lower, handier cases. Less frequently used capital and small capital letters, figures and punctuation marks were in the higher, or upper, cases.