What is hierarchy custom setting in Salesforce?

Hierarchy settings allow you to personalize your application for different profiles and users. The interface has baked-in logic that drills down into the org, profile, and user-level (based upon the current user) and returns the most specific or lowest value in the hierarchy. I’ve found hierarchy custom settings to be extremely useful for those “one-off” occasions. Only this setting is available in Workflow Rules and Validation Rules. This setting is easily fetched in Visualforce: {!$Setup.Setting__c.Field__c}.