Pan flute is an ancient folk instrument. You often see this being played by something mythical, a nymph, unicorn, or minotaur.
This flute is played in a vertical position. Several tubes of graduated lengths are laced together. The varying sizes of the tubes provide different pitches. Each tube gives one pitch, but by over blowing, the player achieves more pitches. This is the same principle as brass instruments use.
Debussy immortalized the Syrinx in his piece for solo flute, Syrinx, La Flute de Pan, the first unaccompanied solo work of the 20th century.
Syrinx was a nymph from Greek mythology, loved by the god, Pan.
In pursuing Syrinx, Pan forced her into a watery death and then mourned his loss.
That mourning is conveyed in Debussy's famous piece, loved by flutists everywhere.
Instructional video...
A lovely ethereal sound, isn't it? An amazing number of styles can be achieved, too... 1) Asian 2) Celtic 3) Blues
Popularity
The pan-style flute, like other folk flutes, is popularity for its ease of playing. You don't need to study for decades to express yourself musically. There is definitely something to be said for that.