Azure Resource Manager

The deployment that uses Azure service management(ASM) are called classic deployment model.
In 2015 Microsoft introduces a deployment model called Resource manager deployment model which is more functional and is replacement for ASM.

Classic and resource manager resources are not compatible with each other. for example suppose you have some VM, storage account classic resources then these can not be seen by Resource manager resources and vice versa. how ever there are techniques which can be used to migrate these resources to resource manager resources.

Please Note:
There are two versions of the portal. The production portal is the Azure portal at
https://portal.azure.com. Most features have been moved to the Azure portal, with some exceptions
such as Azure Active Directory (Azure AD). The previous portal is called the classic Azure portal
(https://manage.windowsazure.com), and it can still be used to manage Azure AD and to configure
and scale classic resources such as Cloud Services.


In Azure every resource should should be in a Resource Group. A resource group is a logical container which holds related resources. in Resource Manager deployment model , the deployment is done on resource group so all the resources in the resource group is deployed in parallel.
Important point here is that all the resource in a resource group should be same life cycle. for example. suppose you have a web application A that uses a database D. if this database is used by some other application also suppose B or database D needs to live after A is retired or you need not to recreate the database D while deploying on A,  in such cases D should be in separate resource group.

here are some points that should be noted for resource groups.

1.  All the resource in a resource group should have same life cycle.
2. Every resource must belong to a resource group.
3. Resources can be added or removed from resource group any time. so if you remove the resource from a resource group then add to other.
4. Resource groups can be used to controll access for the resources there in.
5. Region does not matter for resource and resource groups
6. removing resource group, removes all resources in that group

Why use Resource Manager
With ASM, the deployments were sequential however with Resource manager deploys in parallel so it is much faster that ASM

here is some important points about resource manager,

1. You can create a reusable (JSON) template that can be used to deploy all of the resources for a specific solution in one step. You no longer have to create a VM in the portal, wait for it to finish, then create the next VM, and so on. 
2. You can use the template to redeploy the same resources repeatedly. For example, you may set up the resources in a test environment and find that it doesn’t fit your needs. You can delete the resource group, which removes all of the resources for you, then tweak your template and try again. If you only want to make changes to the resources deployed, you can just change the template and deploy it again, and Resource Manager will change the resources to conform to the new template. 
3. You can take that template and easily re-create multiple versions of your infrastructure, such as staging and production. You can parameterize fields such as the VM name, network name, storage account name, etc., and load the template repeatedly, using different parameters.  
4.  Resource Manager can identify dependencies in a template but allows you to specify additional dependencies if necessary. For example, you wouldn’t want to deploy a virtual machine before creating the storage account for the VHD files that are used for the OS and data disks.  
5. You can use the new Role-Based Access Control (RBAC) to control access to the resources in the group. For example, you can assign the Owner role to a user, giving that user full administrative privileges to those resources in the group but not to other resources in the subscription. Other roles include Reader (you can read anything except secrets) and Contributor (you can do most anything except add or revoke access).  

Resource manager templates are important and they defines deployment and configuration of your application. 

Deployment can be done in a set of template where there is a master template that links all the required template.

Lets talk more about Role Based Access Control(RBAC).

In classic modal either you can grant full administrative permission to a user for all resources in subscription or  no access but in Resource Manager modal you can grant different roles to users or groups at different levels like resource groups or resources.

Roles are actions and notactions which defines what user or group can do and what can not.

there are several predefined roles like owner , contributor etc, 

custom roles can also be created. for more about roles please have a look on the following article on technet.

https://docs.microsoft.com/en-in/azure/active-directory/role-based-access-built-in-roles