Deltiology is the study and collection of postcards. The word derives from the Greek "deltion," which means "small writing tablet." This is a thriving field -- two sites say that deltiology is the third largest collecting hobby worldwide, next only to stamps and coins." Since the hobby took off around 1970, "Dealers' stocks have increased from a couple of shoeboxes of postcards to huge stocks numbering in the hundreds and thousands of postcards," according to articles in Postcard Collector.
Antique postcards, like most ephemera, are very interesting windows into the past; collectors can specialize in particular locations, types of images, types of printing, or artists. "Postcards may contain the only remaining visual proof of a building, person or place; that postcard collections may represent the best images of a natural feature, a historic event or a town at a specific time; and that messages penned on the reverse can reveal a great deal about attitudes of the period." (Mehrer) Indeed, many libraries have special collections of postcards. The early 1900s were the time of a postcard craze which makes cards from this era fairly easy to find. However, many collectors specialize in more modern cards as well.
There are enough postcard clubs (over 100) that there is a Postcard Club Federation and an International Federation of Postcard Dealers, as well as an Institute of Deltiology in Pennsylvania. The IFPD members can sell not only the postcards themselves but boxes, albums, pocket pages, and archival sleeves to store and preserve the cards. Like most collectors, deltiologists grade the condition of postcards from mint on down to fair and poor, and the price can depend on the condition, the age, or the rarity of the card. The stamp or cancellation on a used card may also be of value. But the amount of money something is worth is not necessarily what makes cards so interesting; personal interest in a theme often drives a collector more than investment value.
Novick, Richard. "What is the IFPD?" Postcard Collector Vol. 21 Issue 1, January 2003.
Mehrer, Jim. http://www.postal-history.com/