There are three methods most commonly used by commercial carpet cleaners. I am sure there are more, but the others I have seen are simply variations of the three we will talk about.
The first and most common method used by commercial carpet cleaners is extraction. This method can also be called hot water extraction or steam cleaning. This method sprays a low pressure mixture onto your carpet and the a wand extracts, or pulls back up the solution into a tank. Most people prefer a hot water extraction, that is the water is heated before it is used to clean your carpets.
The second method commonly used by commercial carpet cleaners is called bonnet cleaning. This is where a technician will pre-spray your carpets and then bring a low speed buffer and run this over your floors. This method works well on surface dirt, but cannot get deep down dirt and other foreign matter.
A third method used by commercial carpet cleaners is called dry cleaning. There is a manufacturer that calls their chemical H.O.S.T. You will probably hear people refer to this as the host method. The chemical is actually a damp powder that looks similar to wet saw dust. This is sprinkled over the carpet and allowed to set in for ten to fifteen minutes. Then it is vacuumed up. One of the only advantages to this method is drying time. It will typically take ten to fifteen minutes to dry after the powder is vacuumed up.