A sabot is a specially shaped, two-stage cartridge. It has an outer jacket that helps it travel longer distances, and it has an inner slug or payload. The jacket is designed to fall away in flight after it reaches a certain distance. Several hunting sources suggest that sabot ammunition is only effective at longer distances when shot through a rifled barrel. For a shotgun hunter, this usually means adding on a rifled choke tube.
Sabot can also describe an arrow-like shape of material that fits in a standard shell. One particularly frightening sabot-style payload is the flechette. A flechette round contains hundreds of small, needle- or razor-like projectiles designed to penetrate armor and inflict painful wounds. They are banned by the Geneva Convention but do still see use in combat and counter-terrorism from time to time.