Currently, there are only a handful of states that recognize legal marriage between same-sex couples.
In those states that do allow legal same-sex marriage, same sex couples are entitled to obtain a civil marriage, a union that subsequently entitles them to all the rights and benefits that heterosexual married couples receive from the state, including the right to medical decision-making for partners as well as dissolution procedures akin to a traditional divorce.
However, this is where the rights and benefits end. Federal law does not recognize same sex marriage and in fact, indirectly prohibits it through the Defense of Marriage Act (DOMA). The Act also relieves state governments from honoring a same sex marriage deemed legal in another state.
This means that while you may legally obtain a same sex marriage in some states, the federal government will not honor it. Same sex married couples will not have access to any of the federal benefits extended to heterosexual married couples. This includes joint tax exemptions and credits as well as any spousal claims to governmental programs such as veteran benefits and social security.
In addition, DOMA also allows other states to ignore your legally married status so if you were to move to a state that did not recognize same sex marriages, the state government is not required to honor yours even though you participated in a legal marriage ceremony.