Over the years stamps have served many different functions. A summary of the most common categories of stamps recognized by the large World Catalogues (Michel, Gibbons, Scott, Yvert) follows, with a brief description of the different types.
The most obvious use of regular stamps is as a receipt for postage paid. That is why stamps originally had the same character and seriousness as a banknote or a valuable document. Kings, queens and arms were the dominating motif.
Commemorative (New Brunswick)
Commemorative stamps are produced to celebrate famous persons and special events. There are many suggestions concerning which stamp should be regarded as the first commemorative stamp. One of them is the train stamp (above) issued in New Brunswick,Canada (1860) celebrating the inaugaration of the first railroad.
Pictorial stamp (Sydney views)
The pictorial stamps represent an increasingly popular category. It is easy for the collector to group them together and find a personal favourite, should it be birds, cats, flowers or maps. Unfortunately this has led less scrupulous countries to issue floods of the more popular themes, with the sole intent of profiting from collectors. The first stamp regarded as a pictorial stamp might be the views of Sydney issued in New South Wales, Australia in 1850.
Spinning Jenny (misprint)
Most countries have, at some time, issued stamps to be used on airborne letters. By airmail stamps we usually think of transportation by airplane, but in philatelic history this also encompasses balloon mail, zeppelin mail, carrier pigeon and even rocket mail (!) to name a few variants. The honour of being the first official airmail stamp is usually given the "Curtiss Jenny" stamp (above) issued 1918 in USA. Private and semi-official airmail stamps have however been around since 1877 when a 5-cents label was printed for mail with the hot air balloon "Buffalo" (in Nashville, Tennessee).
Charity (New South Wales)
Stamps sold for more than their nominal value, usually to collect money for a philanthropic cause or organization, are called charity stamps. The first stamps of this type are often considered to be those issued 1897 in New South Wales, Australia. Special 1-penny and 2½-pence stamps were sold for the same amount in shillings, with the difference funding the treatment of tuberculosis.
Official stamp (Afghanistan)
Official (government service) stamps have been produced by most nations to be used by the authorities. Even the first stamp in the world, the Penny Black (issued in 1840), was printed in a special edition with the lettering V and R (Victoria Regina) in the upper corners to denote such official post. However it never came into use. Spain is usually credited with issuing the first official stamp in 1854.
Postage Due (USA)
The postal authorities noticed early on that some letters were not fully franked, due to ignorance or cheating . To recover the amounts, postage dues were printed that the recipient was to redeem. The first postage due was issued in France 1859.
Military stamp (Sweden)
Special stamps to provide free postage for military personnel (in peace or war) are usually called military stamps. As early as 1865 Brasil issued overprinted stamps for this purpose.
Mourning stamp (German Reich)
Stamps have been officially issued for a wide array of other uses as well, including express letters, registered letters, parcels and newspapers, to pay special revenues and to cover different kinds of freight expenses.They have even served for mourning purposes. And that is just to name a few...