Fountains are devices that sit on the ground and emit showers of coloured sparks upwards. They don't shoot effects high into the air - only about ten feet. They provide wide displays of beautiful colours, and can greatly enhance and complement aerial displays.
Larger fountains consist of a combination of multiple tubes fused to go off in sequence. They can easily be confused with cakes by their shape, but your clue that they are fountains is that the fuse is on the top, not the side. They provide much longer lasting displays (from 2-3 minutes!) and emit showers of coloured sparks upto 12 ft high.