What's the difference between postal history and philately, you ask?
Postal history and philately can each be thought of as subsets of one another, depending on how you define them. Philately in general is stamp collecting and everything associated with it: acquisition, research, exhibiting of stamps and stamp-related items. Philately can be technical (stamp issue variations, formats) or topical (who or what is commemorated?). One definition of postal history is mailed envelopes, or "covers", and other objects sent through the mail. People collect postal history because of the information imparted by the objects often tell a story quite beyond the stamps: mailing date and location, where and how it traveled, etc. Collecting postal history is a natural extension of philately. The other way of thinking about it is postal history as the general subject, in the more literal definition of the phrase - everything about the history of the post office. In this sense, philately or stamp collecting is a subset of postal history. Stamps issued by a country is but one part of what a postal service does. The rates, routes, markings, and other postal objects and ephemera are the other related subjects of study here.