An extractor is a person who extracts people's secrets through their dreams. At their core, an extractor is a classic con man – he creates a false set of circumstances that manipulate the subject into revealing his or her secrets. The difference is that those false sets of circumstances are created through dreams.
Dominick Cobb is the self-proclaimed "best extractor out there", as he is hired by corporations with the goal of stealing secrets to which they would otherwise never have access to.
In order to counter extraction, an individual can be trained to resist it at a subconscious level. Robert Fischer had an extractor train his subconscious to protect his secrets, which took the form of militarized projections are trained like real life military and combatants, making it much more difficult for an extractor to successfully extract information from a trained mark.