Three Types of Relief
Classic relief for understatements caused by the other spouse. Separation of liability for divorced or separated spouses. Equitable relief when the other two do not apply but holding you liable would be unfair.
Common Scenarios
Your spouse underreported business income. Your spouse had unreported side income. Your signature was forged. You signed under coercion. Many of these situations can be raised on Form 8857, and a forged signature can mean there was never a valid joint return at all.
The law recognizes that signing a joint return does not make you responsible for your spouse's mistakes.
This guide is general information about federal tax law, not legal advice for your situation. Reading it does not create an attorney-client relationship.