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 are either have finishing strokes, called serifs, like Times Roman, or plain like Arial and Helvetica. Not surprisingly the former are called serif faces, and the latter sans-serif faces. Serifs are not just embellishments in type. They play major roles in the readability, appearance and style of a face. Another type division is between uppercase and lowercase letters, more carryover terms from earlier days when type was set by hand.