Domestic partnerships have different rights and responsibilities accorded to them in different states.

California and Washington register domestic partnerships for same-sex couples, and also for different-sex couples where one person is 62 or older.  Oregon and Wisconsin register same-sex partnerships.  New Jersey registers partnerships where both people are 62 or older, regardless of sex.  Nevada registers couples regardless of sex or age.  Illinois and Hawaii offer civil unions to all couples.  In these states, partners are treated the same as spouses under state law.  

Maine and the District of Columbia register domestic partnerships for couples regardless of sex or age, according them limited rights and responsibilities. In Hawaii and Colorado, couples can register as beneficiaries, again with limited rights.

Domestic partnerships are not treated like marriage under federal law. 

In addition to state-wide registries, many towns and cities have domestic partner registries where domestic partners (defined differently in different places -- most include all couples, but a few are limited to same-sex couples only) can sign an affidavit that they meet certain criteria, pay a small fee, and become "registered" domestic partners. Usually these registries offer no immediate benefits, but other entities -- like employers or insurance companies -- can, if they wish, rely on the registry's information to decide who is eligible for domestic partner health benefits, different insurance rates, etc.       
These registries are not the same thing as domestic partner benefits, which are offered by employers to give employees with unmarried partners benefits similar to the ones married employees receive for their spouses (health benefits, bereavement and sick leave, pension benefits, etc.).

To find out if your locality registers domestic partners, contact the local agency that issues marriage licenses. 

Here are three places to search for local registries:

• City & County Domestic Partner Registries
  This list, divided by state and city, provides information about who is eligible to register, and websites or contact information for each registry for more details.
• City & State Domestic Partner Registries
  A chart by San Francisco's Human Rights Commission, comparing city domestic partner registries as of 2000 (some information may no longer be current, and a great many new registries have been created since then). Chart includes information about how to register, who is eligible, and what, if any, rights the registration grants domestic partners.
• U.S. & Other Countries Domestic Partner Registries
  This list, divided by state or country, from the Partners Task Force for Gay & Lesbian Couples provides information about fees, benefits and responsibilities.

(Last Updated: November, 2008)