it is just wording difference or perhaps have other reasons ?Why are almost all the  glass water pipe called Bong? 

    its is very simple, bong have very negative means and most the time it related to illegal substances usage, when you mention bong it indicated that you smoke weed.  for many smoke shop if you mention you want to get bong ? they will refuse services to you, and ask you to leave, for the same item if you ask you want to get a water pipe, yes, what we can help you sir ? .

  bong and water pipe are referring to the particular kind of item but are just different terminology for use in different situation and different aspect under the law,

Water pipe are created in many different kind of materials , in the beginner of the time, the first one are stone pipe, then bamboo pipe, ceramic, acrylic, and silicone and the most popular one glass pipe,

the glass water pipe ( bong ) are built with interchangeable parts, a bowl for dry herb use, or a banger for oil use, or the body may take apart in the middle. more and more easy to use  and portable to carry and cleans,

the bong have many different filtration  built in the pipe or sometime called peculator , it have two purpose, filter out the tars and create smooths inhale experiences.  there shower head filter, arm tree percolator, and matrix,

example of arm tree peculator.

also water pipe or bong have many it own sub categories, it dividend into recylcer water pipe, a water pipe with many small tube or body that twisted the water flows look so unique  or perhaps weird in others eye.  or  round base pipe or straight body pipe.

people sometime also called a blubber pipe a bong, but that is not entirely accuracy.  water pipe and bubble may have the same function in filtration and better inhale but bubbler are most the time referring to the hand hold glass pipe,  that looks like small hook, made with thick color glass and do not have interchangeable parts,

 simple glass bubbler pipe,

the price for those item are base on the thickness of the glass and how complicate the designs, some top brand item goes to thousands dollars, just remember  more weird more unique looking pipe, cost more money.