What are planetary rings made of, and how do they differ among the four jovian planets? briefly describe the effects of gap moons and orbital resonance on ring systems?

Planetary rings are ice crystals or particles. These particles are numerous and they are small. Jupiter, Saturn, Uranus and Neptune all have rings and are all Jovian planets.

Saturn has big and numerous rings and are visible from Earth. Jupiter's rings are almost invisible and are dusty. Uranus and Neptune have narrow rings but they are very bright and dense and have very thin dust rings in between. Neptune's rings however are more dense than Uranus' and they seem to make arcs rather than complete circles around the planet. 

Gap moons are small moons that orbit within the rings. They cause gaps in the rings because of the gravity of the moons. They push through the ring particles and move them away from each other.

Moons that are further away also affect the rings through resonances. Resonances can launch waves within the rings. Resonances also causes gaps in rings when the moons push on the ring particles repeatedly.