Webhooks are user-defined HTTP callbacks that allow interactions between otherwise independent web applications.
Thorn is a Python framework for building webhooks and event-driven applications. There is also an introduction post with more information on using Thorn.
PyWebhooks is a proof-of-concept library for building webhooks-based services.
Building Webhooks Into Your Application: Guidelines and Best Practices is an extensive high-level guide that defines what webhooks are, why you will want to build them if you need to proactively notify other applications of events, and what security considerations you need to have when creating them.
What's a webhook? is a high-level explanation of this concept that also contains some basic security considerations when using them.
Webhooks for Beginners - Full Course is an entire free video course that shows both how to use and implement webhooks into applications.
Webhooks Provide an Efficient Alternative to API Polling is a high-level overview of the advantages of webhooks over alternatives such as constant polling for updates.