To the uninitiated they look like ordinary envelopes. But to the informed collector who can read the obvious signs like cancel, stamp usage, route markings or any number of factors that make a cover interesting and collection-worthy, the item stopped being a mere envelope the moment it entered the mail stream.
Great Britain issued the world's first postage stamps in 1840. Before that first "Penny Black" was issued, covers with no stamps, but only postal markings to indicate payment and route are known to collectors as "stampless covers."
In 1847 the U.S. issued its first postage stamps: the 5 cent Franklin and 10 cent Washington, which gave birth to the first U.S. stamped covers. The National Postal Museum in Washington D.C. exhibited the largest collection of these covers ever shown several years ago.
The Scott Classic Specialized Catalog naturally starts with this issue on cover in its pricing of relative values of stamps on and off cover. A basic Scott #1 on cover is still priced under $1,000, while number two is more than double that price.
Scott's prices U.S. stamps "on cover" up to the year 1940. This is the cutoff of the "classic" era, according to Scott's. Still there are many worthwhile areas of cover collecting that have yet to be cataloged by mainstream publications.
The so-called "prexies" are some of the last of the U.S. classics. This series of stamps, officially known as the Presidential Issue, released 1938-54, has steadily been gaining in popularity over the years so that covers that until recently were considered common and could have been had for bargain prices are now going for dollars instead of dimes. Those covers from after 1940 fall into the category of "modern postal history" and the value of these items is determined more or less by the market, not any catalog.