Almost every cloud provider exposes a set of APIs to manipulate the virtual infrastructure programmatically. Amazon exposed APIs for storage and compute, which eventually became the core of AWS. Similarly Azure expose Azure Service Management (ASM) API.
ASM was exposed as a set of SOAP endpoints. Other aspects of Azure Service Management is that you just have separation of duties. As you start to integrate more and more services together, ASM becomes increasingly difficult to manage because the components are all separate from each other. When you’re using the classic portal, you’re using Azure Service Management.
By contrast, Azure Resource Manager is a completely re-architected Microsoft Azure. ARM uses the more efficient JSON-based API for development and the resources that you can deploy are much better integrated, and they’re templated so you can use this JavaScript Object Notation format, JSON, to create Resource Manager templates to ease complex deployments. That’s a big time-saver right there. You have separate modes in the cross-platform CLI, depending upon whether you’re doing ASM or ARM. There are even separate PowerShell modules for ASM & ARM.
Things like your website, your cloud database, your storage account and your storage settings. These can all be bundled together and managed as a group, and you can even add your own taxonomic tags. This is especially helpful when you’re concerned about multitenancy.
Finally, Azure Resource Manager is a role-based access control, which makes it a lot easier to delegate administrative authority. With ASM, either you’re the main administrator or a co-administrator, but as far as sub-administrators, it just wasn’t there. It wasn’t thought of.
The original Windows Azure Management Portal was built on top of ASM. With the introduction of Azure IaaS V2, Microsoft introduced a new portal that aligned with the ARM strategy. To differentiate these two interfaces, Microsoft started calling the old portal as the classic portal.
To learn more about ARM & ASM ,see Azure Resource Manager vs. classic deployment
