What exactly are "rights"?

I am choosing to do my expanded definition on the term "rights". I recently had a conversation with a co-worker who said that he has recently spent some time thinking about what exactly a right is. He had been thinking about this after the elections with the California Proposition 8 being passed. This banned gay marriages. So this co-worker was relating to me that there are a group of people who say, "We have the right to marry who we want to" and that got him thinking about what exactly that means. I did some quick Wikipedia searching and found that this is a huge topic as well as an interesting one. Rights can be divided into different categories like civil rights and natural rights (for those that believe that some rights are given to us inherently by nature). The U.S. Declaration of Independence makes reference to "inalienable rights" and those against animal cruelty speak of "animal rights." I think this provides much opportunity for classification and division to attempt to define what rights are, or, at least, to show that the term is highly subjective and has different meanings to different people.