4walls - I think your question is a great one. It made me realize I don't know the difference either. So, I googled around. Here is what I found.
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Same-sex marriage” means legal marriage between people of the same sex.
* Massachusetts issues marriage licenses to same-sex couples (since 2004). On May 15, 2008, the California Supreme Court ruled that same-sex couples have a constitutional right to marry. When the ruling goes into effect in June 2008, California will be the second state to legalize same-sex marriages.
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Civil union” is a category of law that was created to extend rights to same-sex couples. These rights are recognized only in the state where the couple resides.
* Vermont (since 2000), Connecticut (since Oct. 2005), New Jersey (since Dec. 2006), and New Hampshire (since 2007).
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Domestic partnership” is a new category of law that was created to extend rights to unmarried couples, including (but not necessarily limited to) same-sex couples. Laws vary among states, cities, and counties. Terminology also varies; for example, Hawaii has “reciprocal beneficiaries law.” Any rights are recognized only on the state or local level.
* Statewide laws in California, Hawaii, and Maine, Oregon, Washington, and district-wide laws in the District of Columbia, confer certain spousal rights to same-sex couples.
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From
GLAD:
What is a civil union?
A civil union is a legal status created by the state of Vermont in 2000. It provides legal protection to couples at the state law level, but omits federal protections as well as the dignity, clarity, security and power of the word “marriage.”
What are some of the limitations of civil unions?
Civil unions are different from marriage, and that difference has wide-ranging implications that make the two institutions unequal. Here is a quick look at some of the most significant differences:
Portability:
Marriages are respected state to state for all purposes, but questions remain about how civil unions will be treated in other states. GLAD believes there are strong arguments that civil unions deserve respect across the country just like marriages. But the two appellate courts that have addressed the issue (in Connecticut and Georgia) have disrespected them based on the fact that their states do not grant civil unions themselves.
Ending a Civil Union:
If you are married, you can get divorced in any state in which you are a resident. But if states continue to disrespect civil unions, there is no way to end the relationship other than by establishing residency in Vermont and filing for divorce there. This has already created problems for some couples who now have no way to terminate their legal commitment.
Federal Benefits:
According to a 1997 GAO report, civil marriage brings with it at least 1,049 legal protections and responsibilities from the federal government, including the right to take leave from work to care for a family member, the right to sponsor a spouse for immigration purposes, and Social Security survivor benefits that can make a difference between old age in poverty and old age in security. Civil unions bring none of these critical legal protections.
Taxes & Public Benefits for the Family:
Because the federal government does not respect civil unions, a couple with a civil union will be in a kind of limbo with regard to governmental functions performed by both state and federal governments, such as taxation, pension protections, provision of insurance for families, and means-tested programs like Medicaid. Even when states try to provide legal protections, they may be foreclosed from doing so in joint federal/state programs.
Filling out forms:
Every day, we fill out forms that ask us whether we are married or single. People joined in a civil union don’t fit into either category. People with civil unions should be able to identify themselves as a single family unit, but misrepresenting oneself on official documents can be considered fraud and carries potential serious criminal penalties.
Separate & Unequal -- Second-Class Status:
Even if there were no substantive differences in the way the law treated marriages and civil unions, the fact that a civil union remains a separate status just for gay people represents real and powerful inequality. We’ve been down this road before in this country and should not kid ourselves that a separate institution just for gay people is a just solution here either. Our constitution requires legal equality for all. Including gay and lesbian couples within existing marriage laws is the fairest and simplest thing to do.