Question: What's the difference between an "injured spouse" and an "innocent spouse"?
Answer :

From the viewpoint of the IRS, an injured spouse and an innocent spouse are quite different. You may qualify as an injured spouse if your joint income tax refund was held back and applied toward your spouse's past due liability for certain debts, including defaulted student loans, taxes, or child support. In contrast, you may qualify as an innocent spouse if you signed a joint income tax return but were not aware that your spouse understated the tax liability.

To qualify for injured spouse relief, you must report income (such as wages) and make payments (such as federal income tax withholding or estimated tax payments) regarding your joint return. Part or all of your overpayment must have been applied to your spouse's past-due debt. If this is the case, you may be eligible to claim your share of a refund by filing Form 8379 (Injured Spouse Claim and Allocation). On this form, you break down your allocable share of items reported on your joint return. The IRS then calculates the refund (if any) that you are due.

You and your spouse are individually responsible for the entire tax due on your joint return. However, if taxes were understated because your spouse omitted income or claimed improper deductions or credits without your knowledge, an exception may be made: You may qualify as an innocent spouse. You may be entitled to innocent spouse relief if you did not know (and had no reason to know) of the understatement of tax and if, considering all the facts and circumstances, it would be unfair to hold you responsible for the understatement.

To request innocent spouse relief, you must file Form 8857. If you do not qualify for relief under the general innocent spouse rule, other forms of relief (such as equitable relief or the separate liability election) may still be possible. See IRS Publication 971 for details, or consult a tax professional.