Ich or Whitespot Disease is perhaps the most common disease encountered by freshwater aquarists everywhere, both beginning and advanced. It is caused by a protozoan parasite with a complex life cycle - in order to understand how to treat it successfully, you will need to understand a few things about this life cycle.
Low levels of Ich usually result in fish that scratch against inanimate objects in the tank, such as gravel and ornaments. Research has shown that fish can develop partial immunity to low levels of pathogens in their water and establish a tentative balance. Scratching itself should not be cause for real alarm, because some fish can live for years in this state if otherwise kept in optimal conditions, but they should be closely observed just in case. Sudden changes in their environment, such as temperature/pH fluctuations, bad water quality, overcrowding, etc. can disrupt this delicate equilibrium and spur on serious outbreaks of disease (these conditions cause stress, which suppresses the fish's natural immunity). Of special note here are tanks that have not completed cycling (only just set up, but which don't have established nitrifying bacteria in the filter yet), tanks that were overstocked almost overnight, and cases of sudden changes in ambient room temperature. These are especially common in the fall and winter, when ambient temperatures can change so fast that the heater's thermostat lags behind and gives your fish "the chills". Note that, in all these scenarios, Ich should be understood as a consequence of poor fishkeeping conditions, and there´s no point in worrying about treating this consequence without first attacking and correcting the cause of the poor conditions in which the fish are living.
An outbreak of serious Ich will look like white grains of salt on the skin of your fish, from 0.3 to 1.0 mm in diameter. Each individual spot is actually an adult parasite known as the Trophont, which has been enveloped in the pus and tissue of the infected fish's skin and slime layer. Heavy infestations can be very dangerous if they reach the gills or cause secondary bacterial infections, so Ich is definitely not a disease to be treated lightly. If allowed to progress to the point where the fish is very ragged in appearance and hanging near the surface, displaying a lot of respiratory distress, it can be fatal.
The white spots, however, only indicate one stage of Ich (the only one that can be seen by the naked eye). After infecting the fish, the adult organism falls off into the gravel and becomes encysted in a free-living dormant stage known as a Tomont. THIS CYST STAGE IS INVULNERABLE TO MEDICATIONS. This is the reason why a raise in temperature is suggested IN CONJUNCTION with Ich treatments...it speeds up the life cycle and makes the stages that are treatable come around faster. If you do raise the temperature, do so very slowly so as not to stress the weak fish further. Raise it no more than 1-2°C every day, and do not allow this temperature to fluctuate. Also, consider the types of fish you are keeping...most tropicals can tolerate as high as 30-31°C, but most Goldfish will start to languish in the high 20's, so don't push them any further.
Depending on the water temperature, the encysted stage will take from several hours to several days to divide into 200-800 larvae, called Theronts. These must find a host fish within a short window of time or die. They usually infest fish when they scratch against the gravel, continuing the cycle of disease. Usually, all 3 stages will be present within the same tank, but the larval Theront stage is the one that medications and treatments really target.
Luckily, there are a number of very effective treatments available on the market. Some of the most popular include:
QuickCure by Aquarium Pharmaceuticals
Coppersafe by Mardel Laboratories
Super Ick Cure by Jungle Labs
If used correctly, most of these meds have a high success rate, but keep in mind that they are strong treatments that will probably kill your plants (if you have planted tanks, remove fish and treat them separately). Also, with malachite green based meds such as QuickCure, be extra careful and halve the dosage for Catfish and Tetras, which are extra sensitive to the ingredients (and can easily die before they are cured). Treat all fish for at least a week, even contrary to the directions on the label, if you want to prevent reoccurrence later. As can be seen from the complex life cycle, Ich is a tenacious disease that is difficult to eradicate.
Some recommend the addition of aquarium salt (NaCl salt, not to be confused with Marine salt, which contains buffers and should not be used on FW fish) at the concentration of 1 tablespoon per 20 liters with this treatment. This low level of salt (not to be confused with true brackishness) will serve to help wounds heal faster by hyperosmalarity, add electrolytes to help decrease osmotic stress, and also discourage the growth of the parasite. However, salt should be used with extreme caution, since not all fish will tolerate its addition. In particular, do not use salt with sensitive soft water Tetras such as Neons, Cardinals and Glow-Lights, scaleless Catfish (which can be easily burned by it if salt is not pre-dissolved) or live plants.
Through all treatments, temperature should be kept as steady as possible (once it is raised to speed the life cycle), and water changes should be done regularly just as they normally would be (keeping the dosage constant by replacing the amount which is lost with each change). Carbon should be removed from the filter to keep it from absorbing the medication.
With good prevention techniques - buying only healthy fish from reputable LFS, quarantine, keeping the water quality high, and sticking to rules of not overstocking - and some luck, you may never see another case of Ich again.
NOTE 1: There are literally hundreds of diseases that can strike down the fish we keep in home aquaria. Just because your fish is sick, does not mean that they necessarily have ich - fish diseases can have protozoan, macroparasitic, fungal, bacterial, viral, genetic, environmental, and other causes, and all are treated differently. Unless your fish is exhibiting the characteristic salt grain spots, there is no reason to jump to the conclusion that Ich is the cause and start treating for it.
NOTE 2: I have seen posts arguing that Malachite Green increases in toxicity to fish as the temperature increases. You may want to reconsider your decision to use Malachite Green if you intend to raise the temperature at the same time, or if you already maintain your temperature at a higher level than normal. For the record, I have not personally experienced this problem.