The difference resides in the use of the vocabulary. Values can not be decided upon in an arbitrary manner.
In his Two Treatises of government, Locke states that it is people's very own nature which endows them with rights. Under these circumstances, civil society can be considered to exist before the birth of the state. It is society which guarantees the legitimacy of the state and which guarantees a principle of order. The state is a mere instrument through which justice is being done.
When agreeing to the social contract people endow a single authority with an overwhelming power. This authority will make sure that everybody benefits from an impartial justice. Life, liberty and property are the most important rights that the new authority has to protect. In case of a conflict, people will have to make sure that the just principles win.
The role of the government for example is to guarantee the protection of the people's rights and liberties. When the government fails to perform this duty, then it is the right and duty of the citizens to no longer offer their support. This is one of the most important aspects upon which Locke and Hobbes disagree.
The latter one firmly believed that the state ought to have unlimited power. In his opinion this was the only way in which the state could actually put an end to all the existing conflicts. Everything that happens outside the state can only have negative consequences upon people because the free will of people can only lead to conflict. Locke on the other hand believes that the role of the stare is a limited one and it is the very nature of the social contract the one which limits it.
Jean Jacques Rousseau will bring a new perspective regarding the social contract in 1762 with his work, "The social contract." According to him the most important factor that needs to be taken into consideration when finding a reason to support the entire concept of social theory is represented by the popular sovereignty.
Unlike the other philosophers he focuses on the absolute rights that people have in order to decide upon their own lives. From this point-of-view it could be argued that Rousseau is a supporter of direct democracy. In his opinion, popular sovereignty is an indivisible characteristic. In addition, people's sovereignty is an unalienable right. When trying to define this very important concept of sovereignty, one could state that it is a manifestation of the general will. Through general will Rousseau understands more than the mere sum of the wills of the individuals.
The reason to support this conviction resides in the very nature of the person which is selfish. Since people are egoistic and tend to follow their own interests, the only possible outcome would be one of conflict. This is why the individual must be considered as part of a bigger body, that is society. It is only under these circumstances that the individual is able and willing to give up his egoistic will in order to contribute to the existence of a common will.
This common will can actually be considered sovereignty itself. Its role is to decide what is good not for each and every individual as a particular case, but that which is good for society as a whole. This common will is actually the authority to which people give up some of their freedom through the social contract while becoming in return an indivisible part of the whole.
One of the perhaps most interesting phrases that can be found in Rousseau's work which concerns the social contract is that the individuals should be forced to be free. This statement might be considered a contradiction at first sight. If one is truly free that he can non-be forced to do or believe anything. Regardless if freedom is or not a natural state, it is most definitely something that people should always desire and not something that it could be imposed upon them. The true meaning of this phrase is the following one: the individual functions better as part of a bigger whole. Acting on his own, it would be very easy for him to obey his individualist and egoistic impulses.
The only solution is for him to consider himself as part of society and give up his personal will to the bigger general will. Just like it has been stated before, sovereignty is indivisible and unalienable. This is why when an individual fails to act as a citizen, that is a member of the collectivity, society must force him to act as one, because this is the only possible manifestation of freedom.
Law is the tool through which the individual freedom is protected and is in no way a limitation of it. The general will to which the individual gives up his egoistic will use the law in order to exercise its power. Through the social contract the individual aggress therefore to respect the law, a manifestation of the collective will to which he has adhered.
All in all it can be stated that the theory of the social contract deals with important issues such as freedom, while trying to reach the conclusion according to which people are better off living in an organized community than isolated in a state of nature.
Locke vs. Hobbes, Retrieved March 15, 2009 at http://jim.com/hobbes.htm
The social contract, the European Enlightenment Glossary, Retrieved March 15, 2009 at http://www.wsu.edu/~dee/GLOSSARY/SOCCON.HTM