Hang On Back (HOB) Filters are like internal filters except that they are hanged externally on the back.
The advantage of HOB filters is that they are space saving and does not take up space inside the tank. There is also good surface agitation and water circulation because of the water flowing in from the top on the water surface.
The disadvantage of HOB filters is that they usually have very limited space for housing bio media. Also, most of them require using specially made filter pads for mechanical filteration. In the long run, it can be costly as those filter pads are small and does not last long before they need replacing.