And back to the original topic, your friendly mods have posted Snorich's very extensive and helpful explanation about ich in the stickies here and in two other forums.
It's a parasite, rather like fleas. The problem is that treatments that kill this 'flea' outright also kill a tank, so you have to separate the fish from the tank, treat the fish (or just keep changing his water) and let the 'fleas' left behind in the tank starve for want of fish.
Now---I've been at this a Looooonnnnng time. I don't have ich problems. And there's a reason for that. 1. I buy from clean sources. They're spendier, but they take precautions of their own. I don't buy bargain fish. 2. I keep my water quality very high. If a few ich 'swimmers' get past my precautions and do get into my tank, they're going to run into healthy fish who have killer slime coats, and the 'swimmers' are going to die unfulfilled, because they can't infect those fish. The reason the New To The Hobby Forum is an agony-column of "My fish has spots" is principally because people are not perfecting their water quality and keeping their water chemistry spot-on BEFORE they get fish in that tank. Quarantining is major. But even so, accidents can let ich get past your precautions---and if they hit a tank that's got low alkalinity, with the inhabitants' slime coats in a mess and health conditions in a slide---there'll be "I did everything right, I quarantined, and my fish still have ich!" Water quality. Quarantine. Those two things are key to having no ich.