What are Validation Rules and how do they work?

Validation Rules enforce data quality by checking that a record meets specific criteria before it is saved. They use formula expressions that return TRUE when the rule is violated — Salesforce then prevents the save and displays an error message. Key functions used: ISBLANK() for checking empty fields, ISPICKVAL() for picklist values, ISCHANGED() to detect field changes, ISNEW() for insert-only validation, PRIORVALUE() to compare current vs previous values. Tips:

1. Use ISNEW() to apply rules only on creation.
2. Use AND()/OR() for complex logic.
3. Test edge cases carefully as validation rules fire on all save operations including API integrations.