There is no specific type of bag that is referred to as a basketball bag. They can range from a backpack to a duffle bag, depending on player preference and holding space needed. Players will use these bags to carry their personal items to and from practices and games. They will often have them under the bench during games for easy access, or in the locker room if they are not needed during a game.
Higher-level teams will often provide bags for their team members, usually all identical to one another. Some players may choose to use these bags, or they will opt to use their own out of superstition or personal preference.
These bags can hold just about any equipment that a player may need. They should have room for an extra set of clothes, shoes, a mouth guard, a water bottle, socks, and any other game accessories they might use. Many bags will have different compartments to store dirty and clean clothes. There are also specific bags to hold basketballs that a player might bring to a practice.
Since there isn't a specific requirement for what kind of bag a player should have, players can use their creativity and purchase one that matches their style. Basketball bags are available for purchase at sporting goods stores or any retailer online.