A glass jug is made with 7 glass blocks using the cauldron shape and has two main purposes: decoration and liquid storage. It will hold any liquid that a cauldron will hold and is basically a transparent cauldron. Stained glass blocks can be used to make stained glass jugs.
Unlike a cauldron, it can hold one tropical fish bucket, turning it into a fish bowl. Other fish are too large to live in a fish bowl.
If filled with water, it can hold one sea pickle. If empty, it can hold one torch or one redstone torch. Any of these can be used as decorative lights.
(If cauldrons need to be kept distinctive, make it so that cauldrons placed on magma block produce bubbles.)