It's a parasite that attaches itself to fish. At one point in it's lifecycle, it forms cysts that are visible as "salt ". It can occur all over the fish. If it affects the gill area, then the fish, is in real trouble. You'll also see the fish, scratching itself against the tank decorations, ie. Live rock, and anything else you have there.
There's a point in the cycle when the parasite detaches from the host fish, and falls to the bottom of the tank forming a cyst around itself. But then, (and I'm not sure of this part) it comes out of the cyst and can become "free swimming" it needs to find a host within 12 hours or die. Well, in a confined place like a community aquarium, it's easy to do. I lost 6 fish because of an Ich outbreak.
If you have further questions, I would post your question on the Disease and Treatment forum.