Validation rules verify that data entered by a user meets specified standards before a record can be saved. A validation rule contains a formula that evaluates fields and returns true or false. If any data is invalid, an error message displays without saving the record. Validation rules can make fields required based on other field values, ensure numbers are within a range, and enforce date field sequencing. Common formulas used in validation rules include AND, ABS, CASE, CONTAINS, IF, and NOT.