A mule is the hybrid offspring of a male donkey and a female horse.
The donkey father is called a jack, and the horse mother is called a mare. Mules combine traits from both parents: they often have the strength and endurance associated with donkeys and the body size and movement of horses. Their appearance varies according to the breeds used.
Most mules are sterile because horses have 64 chromosomes and donkeys have 62, giving mules 63 chromosomes. That uneven chromosome number usually prevents the chromosome pairing required for normal production of reproductive cells. Rare fertile female mules have been documented, but they are exceptional.
A mule is not the same as a hinny. A hinny is produced by crossing a male horse with a female donkey. Both hybrids are equids, but the direction of the cross affects their typical size, build, and name.