What is Azure Resource Manager (ARM) and how does it differ from classic deployment?

Azure Resource Manager (ARM) is the deployment and management service for Azure, introduced to replace the classic (ASM) model. ARM uses a declarative JSON template approach, supports resource groups for logical grouping, provides role-based access control (RBAC) at the resource level, enables parallel deployment of resources, and supports tagging and dependency management. Classic deployment used separate APIs per service with no unified management layer. ARM templates are idempotent — re-running them produces the same state, making them ideal for Infrastructure as Code workflows.