A set of bagpipes consists of a chanter, which is something like a small shawm, and one or more drones, which are effectively shawms without fingerholes, so that each drone plays only one note. Chanter and drones are plugged into stocks (bits of wood with holes drilled in them). The stocks are sewn into a sealed leather bag, which is made airtight with the application of seasoning compound, so that air can escape only through the stocks, and therefore through the drones and chanter. (Don't ask what goes into seasoning compound. You don't want to know). Another stock houses the blowpipe. When the piper blows into the blowpipe, the bag inflates. When it's fully inflated, a little more lung pressure will start chanter and drones sounding together. That's the reason for plugging them all into the same bag - ingenious, eh?
The piper begins by tuning the drones. He does this by sliding bits of them in and out to make them longer or shorter, until they are perfectly in tune with each other and with the chanter. Now he fingers the holes on the chanter, and you get shawm tunes with drone accompaniment. When the piper runs out of breath, he stops blowing and squeezes the bag with his arm, still playing the chanter. The blowpipe has a simple valve (just a leather flap, but it works), which stops air being pushed back out of the blowpipe, so the sound just keeps going by arm pressure until the piper has breathed in and is ready to start blowing again.
The origin of the bagpipes can be traced back to antiquity. The bagpipes probably originated as a rustic instrument in many cultures because a herdsman had the necessary materials at hand: a goat or sheep skin and a reed pipe.
The bagpipes are mentioned in the Bible, and historians believe that the bagpipes originated in ancient Samaria. It may be possible that even Jesus while attending to his lambs played the pipes. Through Celtic migration, the bagpipes were introduced to Persia and India, and subsequently to Greece and Rome. In fact, a Roman historian of the first century wrote that the Emperor Nero knew how to play the pipe with his mouth and the bag thrust under his arm. During the Middle Ages, the bagpipes were heard and appreciated by all levels of society.
Bagpipes have always been made in many shapes and sizes, and the bagpipes have been played throughout Europe from before the Norman Conquest until the present day. Medieval bagpipes usually had a single drone. (As in contemporary illustrations of Chaucer's Canterbury Tales for English single-drone pipes.) Around 1400, most shepherd-style bagpipes acquired a second drone. A third drone was added after 1550. (See paintings by Brueghel and the illustrations in Praetorius' Syntagma Musicum)
The Renaissance period also saw the advent of small, quiet chamber bagpipes such as Praetorius' Hummelchen or the French shuttle-drone models, some blown with bellows under the arm rather than with the mouth.
The construction of the bagpipes allows a continuous supply of air to be maintained. By squeezing the bag of the bagpipes with the left hand while a breath is taken, the flow of air can be kept up in both the drone bagpipes and chanter. Other features of the bagpipes are the blowpipe and the double reed of the chanter and drone. The blowpipe of the bagpipes contains a round piece of leather hinged onto the bag end which acts as a one-way valve. As a bagpiper blows air in, the flap opens; when the bagpiper stops blowing, the air pressure within the bag of the bagpipes forces the flap shut. The chanter has seven finger holes and a thumb hole (9 notes in total) and has a usual range of an octave and one note.
The bagpipes are ideal for solo dances and monophonic music. Bagpipes have been mentioned for use in polyphony, but if so, problems would arise. The drone would preclude the possibility of any change of mode, and the continuous sound would prohibit observance of rests.
During the Renaissance, the popularity of the bagpipes increased, and the bagpipes gradually moved from country to court. Both Edward II and Edward III had bagpipers at court. King Henry VIII, composer and music patron also had an extensive collection of instruments which, according to a contemporary account, included one with pipes of ivorie and a bagge covered with purple vellat. As a rustic instrument, the bagpipes have been immortalized in the paintings of Pieter Breughel and his contemporaries.
The Scottish Highland Pipes are now the only ones commonly known worldwide. They, and the publicity, are both a product of the nineteenth century. Scotland was not particularly famous for its pipers until comparatively recently. The three-drone Highland pipes were a development of an earlier two-drone pipe - the Irish Great Pipe, or Piob Mhor. This was itself probably a development of an older single-drone pipe.
The modern Highland pipe, and probably its ancestors, were designed as war pipes, the main requirement of which was absolute maximum volume. To achieve these sound levels, they were inevitably made rather raucous, and they require unbelievable amounts of lung pressure. If you're thinking of taking up bagpipe playing, I'd advise you start with anything other than the Highland. You'll be doing yourself and your neighbors a favor.
On the other hand, the Highland Pipes come with a big folk repertoire and a superbly virtuosic technique which is the envy of many other pipers. This piping tradition was also originally Irish. Scots who became particularly adept at piping were sent to "finishing schools" in Ireland. It's perhaps appropriate that Irish Uillean pipes are now becoming better known, thanks to shows like "Riverdance", and a general interest in all things Celtic. My own favorite, though, is the Northumberland pipe, which is the one I would play if I didn't have to remain historical.