Centrifugo is a server built on top of Centrifuge library for Go language.
Due to its standalone language-agnostic nature Centrifugo dictates some rules developers should follow when integrating. If you need more freedom and a more tight integration of real-time server with application business logic you may consider using Centrifuge library to build something similar to Centrifugo but with customized behavior.
Library README has detailed description, link to examples and introduction post.
Many Centrifugo features should be re-implemented when using Centrifuge - like API layer, admin web UI, proxy etc (if you need those of course). And you need to write backend in Go language. But core functionality like a client-server protocol (all Centrifugo client connectors work with Centrifuge library based server) and Redis engine to scale come out of the box.
Many things said in Centrifugo doc can be considered as extra documentation for Centrifuge library (for example part about infrastructure tuning or transport description). But not all of them.