So-called Defense of Marriage laws (DOMAs) define marriage as a relationship between one man and one woman, for the purpose of excluding same-sex couples from the institution of marriage. There are two types of DOMAs: federal and state.
The federal Defense of Marriage Act (DOMA) was enacted so that the federal government could restrict marriage to a union between a man and a woman. In so doing, the federal government was able to deny same-sex married couples any of the federal benefits opposite-sex married couples received.
In June 2013, this all changed - in U.S. v. Windsor, the U.S. Supreme Court struck down DOMA's federal definition of marriage as unconstitutional. As a result, same-sex married couples will be recognized by the federal government and qualify for federal benefits, including immigration status, Social Security benefits and federal tax benefits. This decision does not apply to same-sex couples in domestic partnerships or civil unions.
The Supreme Court did not clarify, however, whether same-sex married couples living in non-recognition states (states that don't allow gay marriage) would qualify for all federal benefits across the board. As of this writing, a few federal agencies, such as the Social Security Administration, continue to determine eligibility for benefits based on where a couple resides, not where they were married. So, for example, in order to qualify for Social Security benefits on a same-sex spouse's work record, the married couple must reside in a recognition state.
Immigration status, federal employee benefits and military benefits, on the other hand, do not depend on where the couple lives. All legally married same-sex couples that live anywhere in the U.S. will qualify for these federal benefits. And, now the same rule applies to federal tax benefits. In August 2013, the U.S. Department of the Treasury ruled that the IRS will treat all legally married same-sex couples as "married" for federal tax purposes - regardless of where they live. Same-sex married couples are free to move about the country without worrying about changing their federal filing status.
Individual states' DOMA laws provide that those states do not allow same-sex marriage and, often, do not recognize same-sex unions from other states.