What Are the Uilleann Pipes?:
The Uilleann Pipes, sometimes known in English as the Union Pipes, are a traditional Irish form of bagpipes. Unlike the better-known Scottish Highland Pipes, which are inflated when the player blows into the bag, the Uilleann Pipes are inflated by a small set of bellows held under the player's arm. These particular pipes are tuned to a specific key (usually D, which is also a common key for the Irish diatonic button accordion to be tuned to, and which also happens to be a key that lends itself well to fiddles -- no coincidence here), but the pipes are chromatic, in that the player can play all full and half notes within a two-octave range.
What Do They Sound Like?:
The Uilleann Pipes are notably quieter and sweeter in tone than the Scottish Great Highland Pipes (also known historically as the Great Irish Warpipes), as the former have always been used for playing music, whereas the latter were used in outdoor settings (on the battlefield, primarily).
How Do They Work?:
Structurally, the Uilleann Pipes work like most bagpipes do: there's a pipe bag (where the air goes), a bellows (the musician squeezes these under their elbow to make air come in), and a chanter (which resembles a recorder, and which the musician uses to finger the melody, and which the air flows through to play the tune). There are also usually two or three sets of drones, which play a constant note (usually the root of whatever chord the pipes are tuned to), and the regulators, additional pipes with which the player can create chords.