Married filing jointly is one of two filing statuses available to married taxpayers, the other being married filing separately. Married filing jointly allows two married taxpayers to file one joint return for both of them.
Generally speaking, filing a joint return results in a lower tax burden thanks to a higher standard deduction and more generous eligibility for other tax benefits. In some specific cases, however, it might make sense to file separate returns.
Your marriage status on the last day of the year determines your marriage status for the entire tax year. If you got married on December 31, 2011, the IRS considers you married for the whole of the 2011 tax year. Likewise, if you got divorced on the last day of the year, you cannot file as married. The exception to this rule is a spouse passing away during the year - the surviving spouse may still file married filing jointly for that tax year.