Barrel pleating is a rarity found on certain kilts. It is a round pleat, rolled, with no sharp edge. On a well done barrel pleat, and the right tartan, you can get an optical illusion effect, looking at it from one angle you might see one colour, and from another angle, a different colour. Instead of being flat, the back of the kilt has rounded ridges.

From what little I know, barrel pleating takes up a LOT more fabric, because you roll a lot of fabric in to the roll to get the proper thickness and weight in to the pleat to make it swing.