Business rules is a new feature added to Dynamics CRM 2013. It allows power users and nondevelopers to create automated processes without the use of custom code.
With the use of business rules, we can now apply form's logic to replace some of the JavaScript code that we previously used for customization. For situations where you want to validate fields as well as show or hide fields based on the values of another fields or for simple form manipulations, we can now implement business rules.
The main reason why business rules were introduced is to simplify the ability of system customizers to make changes to an organization without the need for a developer. While workflows allowed a nondeveloper to customize the system in a wizard-driven mode, they are used only on the server side. Nothing was available on the client side before this feature was introduced, and client-side behavior is very important when customizing forms for users.