Hemp is probably the world's most-bought bagpipe-related commodity.

It is used to seal the joints of the bagpipes, and comes in three forms: plain, black waxed and beeswaxed.

Plain hemp is the old standby, the cheapest and for some reason is always dyed bright yellow.
Black waxed hemp has only existed in the last couple years. It used to be that you would have to take a scrap of leather, wrap it around a gob of black cobbler's wax and then rub it up and down plain hemp (vigorously) until it was completely coated.
Beeswaxed hemp is also fairly new. The process to create it before the pre-waxed stuff was invented was very similar to that for black waxed hemp.

Both types of waxed hemp have the advantage of being much less slippery than the plain hemp. They also keep moisture out better, so the hemp is less likely to expand and crack your bagpipes.