Azure Resource Manager (ARM) is the deployment methodology/strategy to deploy your Azure components in Azure (IaaS and PaaS components). It acts like container of multiple resources however it can span across regions and services. It is template driven, declarative and idempotent in nature.
Following are the benefits of using Azure Resource Manager (ARM) over Classic Services (Cloud Services):
You can deploy, manage, and monitor all of the resources for your solution as a group, rather than handling these resources individually. You can repeatedly deploy your solution throughout the development lifecycle and have confidence your resources are deployed in a consistent state. You can use declarative templates to define your deployment. You can define the dependencies between resources so they are deployed in the correct order.
You can apply access control to all services in your resource group because Role-Based Access Control (RBAC) is natively integrated into the management platform. You can apply tags to resources to logically organize all of the resources in your subscription. You can clarify billing for your organization by viewing the rolled-up costs for the entire group or for a group of resources sharing the same tag.
