A basketball net is a common piece of equipment that is attached to rim on a basketball hoop. They are cylindrical, as they must hold to shape of each rim and hang downwards from the bottom of the rim.
Most nets are between 15 and 18 inches in length and are commonly made of nylon, cotton, polyester, or chain metal. They are usually white to contrast the darker shade of the ball. Basketball nets can be purchased at almost any store that carries sporting goods or basketball equipment.
Basketball nets have been a part of the game from a very early point; the nets themselves were originally closed at the bottom so that players had to climb up to get the ball. However, nets were modified in the early 1900s so that the ball could fall through the net with ease. The purpose of basketball nets is to provide a visual aid for players, fans, and referees to know when the ball goes through the hoop. The nets also provide the iconic swish sound.
Sometimes a chain is used in place of a net on outdoor courts. Chains are more durable and better resistant to the elements of the outdoors. All indoor courts will usually have nets, though.