What are "soft corals" and/or "hard corals"?

[civileng68]

Beside the obvious of soft and/or hard, what makes them different?
I was just at my fish store (sometimes I feel too stupid to ask there) and I saw a tank with lots of corals but, they all looked soft to me in a way. They all moved with the water, so I assumed they were all soft.
Can someone help me out here?

 

[fedukeford]

one difference that i kno of is that if a soft coral dies it will totaly disinigrate leaving just the rock.. if a hard coral dies it will turn bright white and just stay there

 

[viper_930]

What I usually call soft corals are the corals with no solid hard calcium skeleton. When they die and decay, nothing is left behing. Some examples are xenia, star polyps, mushrooms, and zoanthids.
Then there are LPS corals, which have a hard skeleton underneath covered by the soft tissue. Frogspawns, hammers, brains, acanthastrea, micromussa, favias, blastomussas, goniopora, aveopora, and a few more are all LPS corals. When they die the skeleton is left on the rock, but the soft tisse decays away.
SPS are what I associate with the name of hard or stony corals. They look almost all skeleton, with very small soft polyps extending out. This includes acroporas, montiporas, and porites. When they die the very thin tissues and polyps decay, leaving the dead coral in almost the exact same shape as when it was alive.