The level of roast in a coffee is determined by time and temperature—the heat of the coffee roaster and how long the beans are left in. The art of coffee making requires using the precise level of roasting that best brings out the flavor of a particular bean.
Medium (or Viennese) roast coffees are roasted for a longer time or at a higher temperature than light roast coffees in order to caramelize the sugars in the bean. This gives the coffee a note of caramel, a deeper flavor profile, and greater body without obscuring the "taste of place."
If you compare the flavor of beans from the same place of origin—one lightly roasted and one medium roasted—you'll be struck by the different balance of body and flavor that is achieved. Enjoy your adventure.