Community General Hospital is the main set for the show. It is six to seven floors depending on the episode. It holds about 400 beds, with three trauma rooms, two psych wards, and one Intensive Care Unit.
Dr. Mark Sloan is Chief of Internal Medicine. The Marriott Hotel in Woodland Hills, California was used as the exterior of Community General Hospital in the final three seasons of the show (it was also used as a casino in the pilot for CSI).
Here you will see Mark's office (two different offices were used between the earlier seasons and the later seasons), the coroner's office and the nurses' station as the main areas of the hospital seen - but you will see patient's rooms, treatment rooms in the ER and occasionally an OR.