Hobbes
Thomas Hobbes based his theories on government on his belief that man was  basically greedy, selfish, and cruel.  In his book, Leviathan, Hobbes states that life would be a state of constant warfare without a strong government to control man's natural impulses. He believed  people would enter into a Social Contract to escape from this.  In the Social Contract, people would exchange most of their freedoms for the safety of organized society.  Once people entered into this contract, there was no release.  Hobbes did not believe in revolutions, and supported the idea of absolute monarchs.

Locke
Locke also based his theories on his assessment of human nature.  However, Locke believed that people could be reasonable and moral.  In his book, Two Treatises of Government, Locke explained that all men have Natural Rights, which are Life, Liberty, and Property, and that the purpose of government was to protect these rights.  Furthermore, Locke states that if government does not protect these rights, and becomes bad for the people, then they have a right to revolution.  Locke supported a limited government that protected people's natural rights.

Montesquieu
Baron de Montesquieu was an Enlightenment thinker from France who wrote a book called, The Spirit of the Laws in 1748.  In his book, Montesquieu describes what he considers to be the best government.  He states that government should divide itself according to its powers, creating a Judicial, Legislative, and Executive branch. Montesquieu explained that under this system each branch would Check and Balance the others, which would help protect the people's liberty.  The ideas of Separation of Powers and Checks and Balances can be seen in the government of the United States.

Voltaire
Voltaire was a French intellectual who wrote and lectured about freedom of speech.  Voltaire is best known for saying, "I do not agree with a word that you say, but I will defend to the death your right to say it."  He believed that freedom of speech was the best weapon against bad government.  He also spoke out against the corruption of the French government, and the intolerance of the Catholic Church.

Jean Jacques Rousseau
Rousseau wrote a book called, The Social Contract, where he stated that people were basically good, and that society, and its unequal distribution of wealth, were the cause of most problems.  Rousseau believed that government should be run according to the will of the majority, which he called the General Will.  He claimed that the General Will would always act in the best interest of the people.

Impact
Enlightenment ideas helped to stimulate people's sense of individualism, and the basic belief in equal rights.  This in turn led to the Glorious Revolution is Britain, the American Revolution, the French Revolution, and the Latin American Revolutions. Some of these revolutions resulted in government based upon the ideas of the Enlightenment.

Elsewhere, a few monarchs retained absolute control of their countries while also enacting reform based on Enlightenment ideas.  These monarchs are called Enlightened Despots.  In Austria, Maria Teresa and her son Joseph II both introduced reforms based on Enlightenment ideas.  They reduced the tax load on the peasants, provided free education, and ended censorship in their empire.  In Russia, Catherine the Great introduced similar reforms.  She enacted laws for religious toleration and free education, and also sought the advice of nobles and peasants in the running of government. However, these reforms seldom outlived the monarchs who had enacted them.