Utilitarianism, Libertarianism, and the John Rawls theory of Justice, are very similar ideas in that they are rooted in the same primary notions of self-ownership, and uphold the value of morality and rational thought. The most noticeable difference between the three ideas is seen in their individual views on egalitarianism, and the attribution of resources.
Both Libertarianism and Rawls' theory can be understood as subsets of Utilitarianism, as they apply the basic Utilitarian theories posed by John Stuart Mill and Jeremy Bentham, then take these ideas a bit further, albeit in different directions. The basic proposition of Utilitarian thought, as presented by Mill, was that man should act so as to maximize good (pleasure) in the aggregate. Unfortunately, this theory tends to lead to a justification for "the greatest good for the greatest number", and thus to the tyranny of the majority.
Rawls modified this idea by saying that the goal, rather than "pleasure" for the majority is the minimization of "pain" for the most possible. Actions are judged as good or bad depending on the consequences they have for the individual and for others. Thus, Rawls' theory guards against the abuse of the Utilitarian ethic.
Both Utilitarianism and Rawls' theory however, are rooted in the idea of the "collective", eg the "majority" or , the "mob". Libertarianism, while still pulling from the important ideas of free market society and capitalism, seems to more thoroughly understand that decisions made by the "mob" inevitably lead to the undermining of an individuals self-ownership; as part of a collective, the individual does not own himself, rather the collective owns him.
So, Libertarianism poses a philosophy based on private property of the individual, and so, in this theory, freedom is not limited to the "majority", but to any individual who refuses to be enslaved at the whims of any other individual or group.
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