A Custom Script Extension is a piece of software that runs configuration tasks or downloads preconfigured scripts to a virtual machine and then runs those scripts. Custom Script Extensions are typically used to automate the configuration of server settings after a virtual machine has been deployed. Extensions can also be used to install software automatically and to handle various management tasks.

Scripts used by the extension can be downloaded from Azure Storage, GitHub, or can even be provided to the Azure portal when the extension is running. This section will cover the use of Custom Script Extensions when configuring a virtual machine in Azure.

The Custom Script Extension itself works with both Windows and Linux virtual machines and can be run via PowerShell, the Azure CLI, and even the Azure Portal. It can also be called from the Azure Virtual Machine REST API. We'll be covering the use of Custom Script Extensions in a Windows environment, using PowerShell, in this tutorial.