Fastly is essentially Varnish as a service. Similarly to how Heroku drastically simplifies the running of a web server, Fastly simplifies full-page caching of websites with running with dynamic content.
In a nutshell:
- Every time there is a request for your website, it is routed through Fastly instead of directly to your backend.
- If Fastly has an existing cache for the requested URL, it serves the cached page and your backend isn't touched at all.