Custom settings provide developers a mechanism to deliver application metadata and associate this data at the organization, profile, and user level. Custom settings are cached and provide efficient programmatic access without the cost of database queries. This data can be used by Apex, Visualforce, formula fields, validation rules, and the Force.com Web services API.

For example, here's a formula definition that retrieves the value of a custom setting:

{!$Setup.CustomSettingName__c.CustomFieldName__c}