Erlang is a functional programming language and runtime originally developed at Ericsson for telecom systems requiring high availability, soft real-time guarantees, and massive concurrency. It is built around lightweight processes, message passing, and the actor model, with strong support for hot code upgrades and fault isolation. Erlang fits at the programming language and runtime layer and is widely used in distributed systems, messaging platforms, databases, and telecom infrastructure where reliability and concurrency are critical.