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The App Service is a service that hosts one of five kinds of applications:
 Web Apps
 Mobile Apps
 Logic Apps
 API Apps
 Function Apps

Each app runs in its own app service. When you look in the Azure portal to see your website, you will look for the app service in which it is running. It conveniently has the same name as the app it’s hosting.

An App Service plan defines the capacity and resources to be shared among one or more app services that are assigned to that plan.

The following are some of the criteria you can define when creating an App Service plan.
 Location (such as West US)
 Instance count
 Pricing tier (such as Free, Standard, or Premium) providing distinct settings for a variety of performance and service capabilities:

•  Number of cores or instance size
•  Amount of memory
•  Amount of storage
•  Maximum number of instances
•  Autoscaling options (depends on tier—automatic, manual, or none)

When you deploy your app service for the first time, you specify which App Service plan you want to use. At deployment time, you can select an App Service plan you have created or create a new App Service plan.

With infrastructure as a service (IaaS), you can create your own virtual machines (VMs), deploy your apps to them, and deal with the IIS setup and application pools and so on. Then, every time you change an app, you have to deploy it to all the VMs again. If you scale it out, and you have four VMs or eight VMs, it just becomes more onerous. With IaaS, you are responsible for the continuing management of your service. Using App Service plans enables you to run multiple applications on one set of VMs, even if each of the applications is deployed separately.

For example, let’s say you have five websites and three mobile apps that you want to host. You could run each one on its own VM, which would require 8 VMs. If you wanted redundancy (recommended), that would require 16 VMs. Even if you select small instances, the cost adds up really quickly. Plus, you have to scale each set of VMs separately.

If you could run those eight applications on the same set of two VMs, it would be more cost-effective and easier to manage. This is what using App Service plans does for you. You set up one App Service plan with a specific VM size, number of instances, etc. Then, you deploy the eight applications, specifying the same App Service plan for each one. This results in all eight applications running on that same set of two VMs. You can deploy and update each application as needed—you don’t have to update them all at the same time.

When you create your App Service plan, the resources you requested are allocated for you. When you deploy an app to that App Service plan, it simply deploys the applications to those allocated resources. If you decide you want to have four VMs instead of two, you simply go to the Azure portal and modify the App Service plan, changing the number of instances from two to four. It will create two more VMs and deploy your apps to them for you. If you are using small VMs and want to scale up to medium VMs, you can simply modify the Pricing Tier in the App Service plan, and it will scale up. With web apps running in an app service using an App Service plan, the management is handled for you, and you can easily scale up and out just by changing the settings of the App Service plan.

Source of Information : Microsoft Azure Essentials Fundamentals of Azure Second Edition