Your title doesn't match your question.
If you are married you have two choices.
File as married filing jointly. You include all income, deductions, dependents and credits of both spouses on one tax return.
Married filing separately. Each spouse files a return that lists only their specific individual income, deductions and credits. If you share dependents, each dependent can only be claimed once (and there are tiebreaker rules if you can't agree voluntarily).
Married filing separately almost always results in higher taxes or a lower refund, because some tax benefits are reduced or disallowed for that status. However, by signing a joint return, both spouses take full responsibility for the entire return, including any errors or omissions, so it can sometimes be legally smart to file separately even though you might save money filing jointly.
One key point is that when filing jointly, if one spouse claims itemized deductions, both spouses must itemize, even if they have nothing to deduct (which can also increase the tax bill). If you have deductions between you (like a shared mortgage or property taxes), you can divide the deduction any way that you both agree. Individual deductions (such as, one spouse donates to church but the other spouse does not attend) should be claimed individually.
