Anesthesiologists are physicians who are trained to administer anesthetics - medicines used to block nerve sensation. Anesthesia can be either local to one specific part of a body, like a tooth, regional to block feeling to a larger portion of the body, such as during an epidural for child birth, or general to block sensation to the entire body, resulting in unconsciousness.
Anesthesiologists assist in surgery by determining how much anesthesia is necessary and by monitoring the patient's level of responsiveness and vital signs throughout the procedure. The anesthesia specialist will also bring the patient out of anesthesia and then continue to monitor his/her vital signs post-operation.
Besides assisting in surgeries, anesthesiologists may also treat patients suffering from chronic pain.