Ball-jointed dolls are any doll that uses a ball and socket system to connect the limbs and body parts.  Ball and socket articulation creates a doll with limbs that can be moved and posed. 

Ball-jointed dolls (BJD) rose to popularity with the creation of Super-Dolfie (1999) doll line by the model kit company Volks out of Japan.  The original dolls were sold as a cast resin kit that the customer assembled themselves.  These highly customizable art dolls have spread from Asia inspiring artistic expression in collectors and creators all over the world.