A supertag is a way of grouping like tags together. A smartsymbol is a way of grouping images/animations together. The idea behind the smartsymbol is that you could create a smartsymbol of a pump, it would have animations like green for On, red for Off, flash for Alarm, you could also have text display the name, maybe an image to display if the pump is in Auto. Once you group all these images together as a smartsymbol you can use this smartsymbol through out your application. Then lets say you want to change something in the smartsymbol, once you change it, InTouch will update all the smartsymbols saving you development time.
Although I will say, I tried to use the smartsymbols in version 9.0 and it was so bugged I've never tried to use them since.