Ich, commonly referred to as white spot disease, is basically a parisitic infection that is commonly found on fresh water fish.
Although there are various forms of treatment, including anti-biotics, available, I've found that the best way is with something called Aquarium salt, made by doc wellfish.
This salt is a natural tonic to fish and helps them fight off this potentially fatal disease.
However, the best way to prevent ick in your freshwater tank is through regular maintenance and water changes. I usually change about 25% of my tank's water at least once every 2 weeks, and have not had any type of ich, or other common funguses or diseases, as long as I keep on this cleaning schedule.