Whats The Difference Between The Ice Giants To The Gas Giants? And Why Are The Two Different "types"

whats the difference between the ice giants to the gas giants? and why are the two different "types" next to each other?

Gas planets are composed mostly of hydrogen and helium with a thicker layer of metallic hydrogen. In addition, these gas planets also contain a small solid core.

Ice giants on the other hand, are gas planets made out of heavier elements such as oxygen, carbon, nitrogen, and sulfur. For example, Uranus and Neptune only have about 20% hydrogen and helium in mass, while Jupiter and Saturn both have more than 90% hydrogen and helium in mass.

So far, it’s unknown how these planets came to be but there are several theories on how they formed:

http://www.huffingtonpost.com/entry/gas-giant-planets-formation_55db3a67e4b04ae49703b545