Phoenix is the web framework written in Elixir, conceptually similar to Rails but built on the BEAM virtual machine (the Erlang runtime). Its claim to fame is LiveView — a model where most interactivity happens server-side over a WebSocket, with the framework patching the DOM for you, eliminating most of the JavaScript a typical CRUD app needs.
Phoenix shines for high-concurrency, long-lived-connection apps: chat, collaborative editing, financial dashboards, IoT control planes. Discord's real-time backend is on Erlang/Elixir, WhatsApp's historical scale was on Erlang, and several large messaging products run on Phoenix specifically. The framework's niche is real but small compared to Rails, Django, or Laravel.