From my point of view, postal history is the collecting and study of any artifact that demonstrates the handling of mail. My collecting interests are similar to what most postal history people collect: covers and postcards. There are, however, many other areas to collect, such as schedules of Railway Post Office trains, actual mail canceling machines, mailbox locks, uniforms, and so on. For the purposes of this WWW page, we'll stick with the many, many ways you can collect covers and cards that both illuminate the history of mail handling and fascinate the collector.
Information about the history of letters, letter writing, and the broader areas of human communication can be left to others to describe and explain.