Stamps proved to be such a wonderful invention that most countries began to issue stamps designed for special uses beyond first class mail. The Scott catalog makes it easy to determine what special use a stamp was designed for as it gives alpha character prefixes to special service stamps. Thus "B" numbers are for Semi Postals (stamps that have an additional charity value added to the face value), "C" numbers are Airmails, "D" are Pneumatic Posts (tubes used to run under cities) and so on. Most foreign catalogs (especially the Michel catalogs used by the German speaking philatelic world) list special service stamps chronologically with regular stamps which makes them a bit difficult to understand. In general, there is considerable overlap between special service stamps and regular issues. Thus you often see Airmail stamps used to pay non air postage and regular postage stamps used for Special Delivery. One of my favorite mixed usages is when you see Revenues used as postage stamps. The Austrians make a big deal about this in their philately with some examples on cover commanding enormous prices, but Americans look upon such varieties of use on our stamps as mere curiosities.
Special Service stamps come in three types. First, they distinguish a different service of mail carriage and the stamp indicates that additional postage has been paid . Examples of this are Air Mail or Special Delivery. Second, they indicate an advertising or charitable intention on the part of the user. Thus, Semi Postals are often used on wedding invitations or birth announcements. And third, they indicate a governmental or postal purpose such as Official mail or Postage Dues.
Great Britain was the first country to issue special service stamps issuing an Official stamp coincidental with the Penny Black. Today many collectors specialize in these stamps as they offer a microcosm of any country's philately that is appealing to many collectors.