Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington, and Wisconsin are all community property states. In addition, Alaska is an "opt-in" community property state.
Generally, the property, income, and assets owned by a married couple residing in a community property state is jointly owned by the couple, with exceptions for property acquired before the marriage or through a gift or inheritance.
However, what is considered community property varies by state and no two community property states follow the same set of rules. Page 4 of IRS Publication 555 provides a general overview.
Couples filing separately in community property states must follow their state's rules when allocating property on their respective returns.