There's more...

Business Rules provide a wide range of actions that can be performed. Among them are the following:

• Show error message
• Set field value (using another field's value, a static value, a formula, or simply clear it; it's good for simple plugin replacements)
• Set visibility
• Set default value
• Lock or unlock a field

The TechNet article (https://www.microsoft.com/en-us/Dynamics/crm-customer-center/create-business-rules-and-recommendations-to-apply-logic-in-a-form.aspx) covers more details about Business Rule.