An adverse action notice is a letter sent after you've had an application - credit, loan, insurance, employment, etc. - denied. If you were denied because of information in your credit report, the Fair Credit Reporting Act (FCRA) requires creditors, lenders, and other businesses to send an adverse action notice letting you know the reason(s) you were you were denied and the name, address, and phone number of the credit bureau who provided the credit report. The adverse action notice can be oral, by phone, or in writing and must be sent in a "reasonable" amount of time.
The credit-based adverse action notice must include a statement letting you know you can obtain your credit report free from the agency listed in the notice within 60 days. The notice will also state that the credit bureau wasn't involved in the decision and that the bureau can't tell you why your application was denied. (Those reasons will be listed in the notice.) Finally, the adverse action notice will include a disclosure of your right to dispute inaccurate or incomplete credit report information.
If you had a credit card or loan application denied, but not because of information on your credit report, you'll still receive an adverse action notice. This adverse action notice will also list the specific reason(s) your application was denied, but won't include the name of a credit bureau or allow you to receive a free credit report. There will be an Equal Credit Opportunity Act (ECOA) notice at the end of this adverse action stating it's illegal to deny your application based on race, color, religion, national origin, sex, marital status, age (as long as you're old enough to sign a contract), participation in public assistance program, or exercising your Consumer Credit Protection Act rights.