Briefly,

John Rawls is best known for his liberal theory of justice, or justice as fairness. Its defining characteristic is its acknowledgment that society is made up of individuals who espouse many disparate beliefs and conceptions of 'the good.' Government of a society which is based on certain conceptions of the good-- religious, moral, etc-- causes problems because people whose views differ end up straight disrespected by law.

To establish the basic rules for the government of society, Rawls discusses what he calls "the original position" (OP). In OP, subjects are placed behind a veil of ignorance. Behind the veil we are not aware of our sex, class, intelligence, physical ability, or anything else that would distinguish us in the normal world. Subjects in OP are granted rationality, self-interestedness, knowlege that resources are moderately scarce, and the knowledge that there are many others under the same conditions and asked to pick the basic rules by which their society should be governed.

Choosing principles behind veil of ignorance ensures they are not tailored to favour anyone in any particular position. The simplest way to look at it is to imagine being assigned the task of cutting up a pie without knowing which piece you're going to get. You want to be certain you get the biggest piece, so you'll cut the pie into perfectly even pieces.

The principals Rawls arrives at are:

1) Each person shall be allowed the most extensive scheme of liberty compatible with a similar scheme for everyone else.

2) a) Offices of power shall be attached to positions open to all. b) Inequality must benefit the least advantaged.

With the basic rights established, the idea is that everyone is able to pursue their conceptions of the good, as long as it doesn't entail hoarding resources or inhibiting others' ends. Rawls's theory of justice is related to Kant's Metaphysics of Morals because of the importance it places on the individual will, each person as an end in him/herself and locating the right prior to the good.