Validation rules versus formula fields for data quality
Key Takeaway
A validation rule blocks bad data at save time. You write its formula to describe the bad condition, and when the formula returns True, the user sees your error message and the record doesn't save. A formula field calculates a read-only value from other fields, so nobody has to type it.
Validation rules and formula fields both use Salesforce formulas, but they protect data quality in opposite ways. A validation rule checks what a user enters and blocks the save with an error message when the data breaks a standard you set. A formula field calculates a read-only value from other fields, so users never type that value and can't get it wrong.
Formulas in both tools
A formula is an algorithm that gets its value from other fields, expressions, or values. The same operators and functions work in formula fields, validation rules, approval processes, and workflow rules, with some exceptions. The two tools differ in what Salesforce does with the formula's result.