Dry cleaning is more of a chemical cleaning than anything, and it is not really dry, some water is used to inject the chemicals into your carpet. The amount of water used is in reality very minimal and it is very quick to dry.
Once the moist chemical is placed into your carpet it is then brushed into the fibers and nap of the carpet, it mixes with the dirt and other debris in the fibers and nap and then it is allowed to dry. As the chemicals dry they crystallize and trap the dirt and debris in small granules and once the carpet dries it is then vacuumed and the dirt is removed in this manner. The cons of dry processes is that they do not deep clean. It only cleans the fibers and unless it has some kind of odor remover it is just surface cleaning at best, carpet does feel and look cleaner after dry cleaning. The pros are that you have very little moisture on your carpet and it dries fast.
Steam Cleaning Carpets is a deep cleaning because it is hot water that is actually pumped into the carpet, it is not actually steam. It saturates the carpet, and the mat of the carpet, mixes with the dirt and debris and is then extracted buy high power vacuum systems.
The pros are that it gets the base of the carpet cleaner and removes more dirt and debris. The cons of steam is that it takes longer for your carpet to dry, leaving the potential for odors if not thoroughly extracted.
Most professionals have top quality extraction equipment and can get the water out for the most part and this makes it 100% safe in this present time because of new designs and increased power in the extractors. Now you have the majority of the facts and can now make an educated decision on which route you wish to go in your carpet maintenance.