This question matches core Azure compute services with their primary functions.

• Azure virtual machines are the foundational Infrastructure as a Service (IaaS) offering, providing complete operating system virtualization.
• Azure Container Instances (ACI) is a service that runs Docker containers on-demand, offering a portable, virtualized environment for applications without VM management.
• Azure App Service is a Platform as a Service (PaaS) specifically designed to host web applications, REST APIs, and mobile back ends, managing the infrastructure required to scale them.
• Azure Functions is a serverless, event-driven compute platform that allows you to run small pieces of code ("functions") without managing the underlying server infrastructure.

Azure virtual machines:

Source: Microsoft Docs - "What is an Azure virtual machine?"

Reference: "Azure virtual machines (VMs) is one of several types of on-demand, scalable computing resources that Azure offers... A VM is a physical computer's emulation, which offers the flexibility of virtualization without having to buy and maintain the physical hardware that runs it."

Azure Container Instances:

Source: Microsoft Docs - "What is Azure Container Instances?"

Reference: "Azure Container Instances (ACI) is a solution for any scenario that can operate in isolated containers, without orchestration... Containers are lightweight and portable... Because containers run directly on ACI, they get the same isolation as if they were running on a VM but without the overhead of a full OS."

Azure App Service:

Source: Microsoft Docs - "Azure App Service overview"

Reference: "Azure App Service is an HTTP-based service for hosting web applications, REST APIs, and mobile back ends... It's a fully managed platform as a service (PaaS) that takes care of the infrastructure... You can easily scale your apps... to meet demand."

Azure Functions:

Source: Microsoft Docs - "An introduction to Azure Functions"

Reference: "Azure Functions is an event-driven, serverless compute platform that helps you build more efficient, scalable, and reliable applications... Functions lets you write code in the language of your choice and it runs that code in response to events... without having to provision or manage servers."