Most female fish produce eggs that are fertilised externally by a male and either scattered or laid on objects. Livebearers however, are fertilised internally by the male, usually through the use of a modified anal fin called a 'gonopodium'.
This allows the development of young to occur inside the protective environment of the parent body. When born, the fry are fully formed and able to swim and function as miniature adults, giving them a survival advantage over egg laying species.
Most livebearing fish are relatively hardy species and do well in aquariums. Common livebearers include guppies, mollies, platies, and swordtails