Hierarchy: Tenant (your identity boundary) → Subscription (billing) → Resource Group (lifecycle unit) → Resources.
IaaS: you manage the OS (Azure VMs). PaaS: Microsoft manages the platform (App Service, Azure SQL (Structured Query Language)). SaaS: you just consume (M365 (Microsoft 365)).
Always tag resources at creation. Untagged = un-billable, un-owned, un-deletable.