A formula is an algorithm that derives its value from other fields, expressions, or values. Formulas can help you automatically calculate the value of a field based on other fields (from Salesforce doc). Formulas can be used in many areas of Salesforce other than just Formula Fields, such as Validation Rules and criteria for Workflow Rules and Approval Processes.
Questions related to designing, implementing, using, debugging, or anything else about formulas should use this tag.
Note that there are limits to formulas in Salesforce, such as size limitations.
See the following for more information: