What is a WebHook in Azure

There isn't any service available in Azure called "webhook". A webhook is simply an addressable HTTP endpoint that allows external applications to communicate with your system. You could implement webhooks using a variety of Azure services such as Azure Functions, a web app running an API, etc.

This is a bit of a late answer but it may help someone.

In Azure you can use webhooks to trigger an Azure function see Microsoft Functions Documentation

A webhook in azure is an HTTP endpoint. It is a user defined address you can call with relevant information to interact with several other services. Think of it as a sort of mailbox that you can configure services to respond to. You send an HTTP request (mail a letter) and it lands in this mailbox and you have configured say... Azure functions to respond to that particular mailbox or … logic apps or … data factory … The last one for example. You can have data factory post to a webhook when its completed its work if you need some follow on function to be notified upon job complete.

These are different from functions or webjobs in that they do not have any programable logic to execute a task or job. Webhooks are a customizable location to which you can post HTTP requests.