A politician is person who is professionally involved in politics. Usually, the word politician refers to someone who has been elected to a political office. Examples of United States political offices would include mayor and county commissioner on the local level; state senator and governor on the state level; and senator, president, and congressperson on the national level. President Obama, for example, is a politician since he is professionally involved in politics as an elected official.
Political thinkers, on the other hand, engage in the study of political philosophy. They consider the nature of government and answer philosophical questions such as "What makes a good government?," "What is justice?," and "What is the proper relationship between a citizen and the government?" Political thinkers may also be politicians (such as Thomas Jefferson and James Madison), but they often are not (such as Plato or Thomas Aquinas).