What is Rousseau's idea of a social contract?

Rousseau's social contract theory was an important concept for Enlightenment philosophy.  The social contract is the idea that government is a necessary evil because individuals in nature act selfishly.  Since individuals think of themselves first, a person's safety and freedom would be in peril without a government to protect it.  The social contract is the idea that individuals sacrifice certain freedoms for the benefit of the protection provided by the government.  The government has a responsibility in this contract too, however.  The government should act in the best interests of the individuals that it represents.  If the government does not act responsibly within this social contract, the citizens have a duty and right to replace that government and form a new contract.  The writings of Rousseau were very influential on the founders of the United States, and Thomas Jefferson, in particular.