What is the difference between a protectorate and a sphere of influence?

A "sphere of influence" is an area in which a foreign power claims to have exclusive privileges (but does not play a formal role in the nation's internal affairs). These privileges may be economic, political, or cultural, and they do not necessarily require the approval of the "influenced" nation. For example, the United States declared a "sphere of influence" over the Western hemisphere when they passed the Monroe Doctrine in 1823. They threatened to view further European colonization of the Western hemisphere as acts of aggression.

In a "protectorate," the foreign power exerts formal control over the protected nation. The protected nation still retains its own internal government, but the foreign power exercises plays an active role in some aspect of its government (foreign policy, for example). The degree of control may vary. A foreign nation may simply guarantee to protect another nation from invasion, or the protected nation may function as little more than a puppet government ruled by the foreign one.