What's the difference between formatting and validation rules in Nintex Forms?

Formatting rules allow you to control things like the visibility of a control, whereas validation rules allows entered data to be validated against other controls. Validation rules are useful if you want to, for example, ensure a field is not left blank if another field meets certain criteria. E.g. Ensure that the description field is not blank if the sum of the purchase order requisition is greater than $5000.