A Fly App is an abstraction for a group of Machines running your code on Fly.io, along with the configuration, provisioned resources, and data we need to keep track of to run and route to your Machines.
Every Machine on Fly.io lives in a Fly App. Anycast IPs, certificates, Fly Volumes, and custom domains belong to apps as well.
Fly Apps run on flyd. Learn more about how flyd works and how it came to be in the blog post: Carving the Scheduler Out of Our Orchestrator.