What is indicated by a white marker buoy with an orange circle and black lettering?

An orange circle means a controlled area, such as a speed limit, anchoring or wake restrictions.

They all look very similar, so you may want to refer to the explanation of more shapes: http://www.boatsafe.com/nauticalknowhow/regmarks.htm

*NOTE that an orange diamond with black lettering within the diamond means a danger exists, such as rocks, shoals, stumps, or dams.