This has nothing to do with the idea of falling back in a battle sense, but merely a signal to mark of the end of the soldier's day - usually around 6 pm. Traditionally, in the Scottish Regiments, the piper plays a traditional "Retreat March" at this time. What defines a "Retreat March" in the musical sense is a march played in 3/4 or 9/8 time - i.e. 3 beats in the bar in simple or compound time. There are many beautiful tunes to pick from, with the repertoire greatly increased during and immediately following the First World War.
The Hornpipe was traditionally an English dance accompanied by a simple single-reeded instrument called a hornpipe comprising a wooden tube with a horn bell on the end. In the last couple of centuries the music and the dance became (for reasons which escape me) associated with sailors. In the Highland Dancing world, the Sailor's Hornpipe has been in the repertoire for as long as I can remember, and the piper played such tunes as "Jackie Tar" or "The High Level Hornpipe" to accompany the dancers. However, in recent years there has appeared a whole swag of "Hornpipes" which have nothing to do with a dancing tradition, but are merely very lively marches which tend to require a bit more than average dexterity in fingering. It is almost mandatory in Grade 1 Pipe Band competitions to march on to a Hornpipe so this craze is not going to disappear any time soon !!! The hornpipe has exactly the same format as a 2/4 march - 2 beats in the bar and simple time.
Reels and Jigs.
These are traditionally tunes to accompany fast and furious dances. However, the Reel is in simple time (2/2) while the Jig is in compound time (6/8 or 9/8). But of course, pipers being what they are, the Reel can now be a dance tune or a "competition piece" with quite challenging fingering and played much more sedately with no hint of its wild origins. It should be noted that some Hornpipes (2/4) can be difficult to tell from Reels (2/2) when the tempo is raised.
This is the name of a traditional Scottish dance which probably originated in the valley (strath) of the River Spey a couple of centuries ago. The music is fiercely "cut and hold" to the point where it is probably unique in the world of music. The music is written in simple time, with 4 beats to the bar. Once again, the Highland Dancing scene is closely associated with the strathspey - e.g. the Highland Fling - but there is also a piping tradition of "heavier" strathspeys used in competition which uses the same unique musical structure to great effect.