An innocent spouse filed a joint return, but was unaware that their spouse deliberately under-reported their tax liability. By filing as an innocent spouse (Form 8857), you can be protected from additional taxes, penalties, and interest when the IRS later finds out about the unreported income.
In contrast, an injured spouse seeks to protect their share of the joint refund in case it gets seized or offset due to the other spouse's debts or unpaid obligations.
How to file Innocent Spouse, Form 8857
How to file Injured Spouse, Form 8379