In Salesforce, validation rules are used to enforce certain data requirements and constraints on records, while formulas are used to perform calculations and generate values for fields on a record.

Validation rules can be used to ensure that certain fields are not left blank, that specific values are entered in fields, or that certain conditions are met before a record can be saved. For example, a validation rule can be created to require that the "Close Date" field on an opportunity record is not in the past.

Formulas, on the other hand, can be used to calculate and generate values for fields on a record. Formulas can be used to perform calculations, concatenate text, or manipulate date and time values. For example, a formula can be created to calculate the "Expected Revenue" of an opportunity record based on the "Probability" and "Amount" fields.

In summary, while validation rules are used to enforce data requirements and constraints, formulas are used to calculate and generate values for fields. Both validation rules and formulas are important tools in Salesforce to ensure data accuracy and automate processes.

One scenario where both validation rules and formulas can be used together is in enforcing a business rule that requires a custom field on an opportunity record to be automatically calculated based on the values of other fields on the same record.

For example, let's say that a company wants to enforce a rule that the "Expected Revenue" field on an opportunity record should always be equal to the "Amount" field multiplied by the "Probability" field divided by 100. This calculation should happen automatically whenever a user creates or updates an opportunity record.

To enforce this business rule, a formula field can be created on the opportunity object that performs the calculation as described above. The formula can be written using the Salesforce formula language and can reference the "Amount" and "Probability" fields on the opportunity record.

However, to ensure that the "Expected Revenue" field is always accurate and consistent with the calculation, a validation rule can also be created that checks whether the "Expected Revenue" field matches the formula calculation. If the validation rule fails, the user will not be able to save the opportunity record until the issue is resolved.

By using both a formula and a validation rule together, the company can ensure that the "Expected Revenue" field is always automatically calculated based on the values of other fields on the same record and that the calculated value is always accurate and consistent with the formula calculation.