When filing an individual income tax form, the IRS uses the following Tax Classifications:
You may file as Single if you are not married, widowed (after some amount of time has passed), or legally separated under a decree of divorce or a separate maintenance contract.
You and your spouse file together, or jointly, using a single income tax form. Both spouses' income and itemized deductions are summarized on the tax forms. Each spouse is responsible for the accuracy of what is reported on the tax forms, and both must sign the appropriate forms.
When married couples file "Married Filing Separately", each spouse completes their own tax form. The rate of taxation may be different than filing jointly, and for some, filing separately can result in reduced tax liability for the married couple.
To qualify as a Head of Household, a person must be single and be responsible for more than half of the costs for one or more qualifying dependents. This status typically will be taxed at a lower rate than filing "Single". Some married couples that live separately may qualify for the Head of Household status. Consult IRS documentation for specifics on this.
To qualify for this special classification, your spouse must have died in either of the 2 preceeding tax years. You must have one or more qualifying dependent children. Other requirements may apply for this special classification, so be sure to check the IRS regulations before filing with this status.