Bag, sack, pouch denote a container made of a flexible material (as paper, cloth, or leather) and open or opening at the top.
Bag is the widest in its range of application and is referable to anything that comes under this general description and is used to hold something.
Sack is usually more restricted in its application than bag; within these limits, however, the terms are interchangeable.
Sack commonly suggests oblong shape, a coarse material and, often, crude workmanship.
It is probably more often used than bag when it refers to containers and their contents.
Pouch is applied chiefly to a small bag carried on the person or in the hand and used as a substitute for a pocket; it often specifically designates a bag that is opened or closed by means of a gathering string, zipper, or flap.