The Medina is the historic area of Marrakech, the millennium-old Arab city (in opposit of the New City (Gueliz) built during the French protectorate. All the monuments, the majority of museums and accommodations are in the Medina. It is the most touristic and charming part of Marrakech. The Medina is a labyrinth of alleys, cul-de-sacs, squares, souks punctuated by riads, palaces, mosques and Koranic schools (madrassah). At first sight, the Medina seems to be an anarchic whole of an inextricable disorder. In fact, it's easier than it seems.
You will spend most or all of your time there.