Bubblers are a smaller size glass rig that don't have as much chamber space as a larger functional glass piece but still have the water filtration that a pipe doesn't. Less about building up a chamber of smoking then inhaling and more about just a constant pull through from bowl to mouth like a pipe. Bubblers come in two variations, standard for most regular smoking bowls and reverse for concentrate accessories like nails and domes.
This is the type of joint you find on most downstems for functional glass or bubblers. Standard joints are female and require a male bowl, adapter or percolator of the same size to fit into it.
The reverse joint is commonly found in oil rigs. The male joint accepts a corresponding sized nail and female dome, banger, domeless nail, adapter or anything with a matching sized female joint. Bowls to smoke dried legal smoking herbs can be found that fit these joints but they are less common.