Web Role is primarily used to run web applications, such as ASP.NET, PHP, Web API, WCF, and others, which runs on IIS.

Cloud service can be deployed as a package and, during deployment, you can specify the roles that you need, choose the target operating system, and scale your service accordingly. Cloud service is managed by Azure Service Fabric, and it controls the environment and the virtual machines that your service is hosted on. So, for example, if you have selected two instances to be running and once instance fails, Azure Service Fabric detects that failure and uses some other rack in the same data center to bring up that instance. It uses the shared storage between multiple VMs, and makes it easy to switch between different VMs in case of any failure.