Ich Ichthyophthirius multifiliis
Ich is an ecoparasite that can harm fish if they succumb to it. If not cared for properly, it can kill fish.
It's important to note that Ich is always present, and there is nothing you can do to get rid of it. Fish are naturally immune to Ich, and only succumb to it when their immune systems or protective slime-coating have been compromised, or when they're stressed.
Think of it like a person with the flu. The flu virus will always be present, and our bodies have natural defenses in place to protect us, but external factors such as lack of sleep, a person who is already infected sneezing nearby, not washing hands, etc. can all cause the virus to get past our defenses and make us sick.
This is why you will commonly see fish contract Ich after traveling, if they're being bullied by other fish, and when they're living in poor water conditions. Also, if you have a fish like a Plecostomus, Loach, or certain catfishes, they will actually attach to other fish, and "bite" off their slime coating, exposing them to Ich.
Finally, just like if you moved a person sick with the flu into a house full of healthy people, introducing a fish with Ich into an established aquarium is a bad idea.
Ich can be treated in several different ways:
Chemical Treatment: Something like Ich Attack or ParaGuard are two that I personally recommend. Follow the directions on the bottle, it'll tell you how much to put in per gallon, and how often. Be careful, these two shouldn't effect anything else in your tank, but with chemical treatments you always run the risk of throwing off the chemical balance (usually Ph levels). Some can also be quite bad for invertebrates, like shrimp and crayfish.
Aquarium Salt: Adding aquarium salt to the water helps calm fish, providing them with certain nutrients, and removes nitrites helping fish to absorb more oxygen. I wouldn't recommend using aquarium salt with Plecostomus in the tank, as they can be sensitive to it, though some people have no problems. Also, aquarium salt is special, don't use other kinds of salt.
Temperature: Raising the temperature to about 86 degrees Fahrenheit will create an environment that the fish can survive in, but not the parasite. Don't leave the temperature at that though, as it will stress the fish out to be in there for long. It's all about who can survive it longer.
Aquarium Salt and temperature: I prefer this option as the salt helps the fish through the temperature rise more. Again, think twice about salt and Plecos.
On top of the treatment, I would perform gentile water changes. Something like 10% every 5 days. Enough to take out a good chunk of the old water, but not enough to stress the fish anymore than necessary. Have a bucket of water prepared the day before with something like Seachem Prime.