Entropy is the degree of disorder.
Systems will move to a state of maximum disorder unless forced otherwise. (E.g. Tidying a room: If a room is not tidied periodically, it will eventually reach a state of optimum chaos)
Diamond has a very ordered structure, so it's least disordered, so it has a low Entropy.
Gases don't have ordered structures, so they're most disordered, so they have a great Entropy.
Solids have the lower entropy.
Liquids have a medium entropy.
Gases have the greatest entropy.