There are five possible filing statuses, based on your marital status and family circumstances, which determine the amount of your standard deduction, your eligibility for certain credits, and your tax bracket, among other things.
Single - For unmarried filers.
Married Filing Jointly - For married couples wishing to submit one joint return, including individuals whose spouse died during the year.
Married Filing Separately - For married taxpayers who wish to file separate return from their spouse.
Head of Household - For unmarried individuals who pay more than half the cost of keeping up a home and who have a qualifying dependent living with them for at least half the year.
Qualifying Widow(er) with Dependent Child - For those whose spouse died during the previous two years and who maintain a home for qualifying dependents.
Note that your marital status at the end of the year generally determines your filing status for the entire tax year.