In this chapter, you have learned about ways to insure the integrity of your all-important data. You can achieve this goal by limiting the values available for entry by a user, either with picklists or relationships, or by eliminating the need for users to enter data by creating the data with a formula field. This last section gives you one more tool for enforcement: validation rules.
Validation rules are defined in much the same way as formulas, with access to the same helpers and functions. But a validation rule works in a more specific way:
Uses an Error Condition Formula that evaluates to either TRUE or FALSE
Prevents the user data action that spawned the validation if the formula evaluates to FALSE
Displays a custom error ...