Roussseau was a philosopher and teacher that defended the social contract: the agreement the people and the governors, sign to preserve social order; and the general will, that consists in the power of the people to decides directly by themselves and not by a representative.
Montesquieu defended the division of powers (executive, legislative and judicial) and an independent judicial power.
Voltaire defended that the power of the king has to be limited by the Parliament, that everybody have to pay taxes (universal tax system) and he critized the church because he thought that the church was intolerant.
Then, she has explained the physiocracy and economic liberalism.
Physiocrats considered agriculture to be the most important economic activity of a country and they supported privated property, free trade, free industry, and they were against any intervention of the State in economy. Quesnay was the most important physiocrat and Adam Smith was an economy teacher in a University and he was the advocate of the economic liberalism.
Finally we have talked about the Enlightened despotism. Some monarchs adopted ideas of the Enlightenment and introduced some reforms in their kingdoms, but as they wanted to preserve their absolute power: they did "everything for the people, but without the people". They didn't touch the structure of power, the privileges of nobles and clergy and also they thought that the people were like ignorant children or they couldn't think, so the reforms they made were insufficient.