I would say it refers to a branch of collecting such items as envelopes, postcards, and wrappers with postage affixed, sold by post offices around the world. Some people collect by countries, states, cities, post offices, cancellations, events, and the list goes on and on. Cover collecting is a wide open field and an extension of stamp collecting. A “cover” is an envelope that has seen postal service. Many have “cachets,” a rubber stamped or printed impression or label placed on the cover descriptive of the event for which it was mailed.

Airmail postal history collectors will browse for First flights, Catapults, Zeppelins, Crash covers, Transpacific Airmail and all sorts of Commercial International Airmail.

Military postal history collectors will browse for war covers, APO covers, U.S. naval ship covers mailed by sailors, patriotic covers and other areas that pertain to military history.

There are so many areas to collect in there is just no way I can list all of them here. I haven’t even started on town cancellations, DPO’s, machine cancellations, flag cancellations, cities, states and many more collecting interest.