The gall bladder is a structure found under the liver of many vertebrates, including humans, where it stores bile that is secreted by the liver. Bile is transported to the duodenum through the bile duct; its main function is to aid in the digestion of fats.
In humans, the gall bladder is sometimes removed as a treatment for gallstones. This procedure is called a cholecystectomy.
The image below shows the gall bladder and the duodenum which are both stained green due to the presence of bile.