Erlang - Layers

Erlang is a language with several layers, each of which are supersets of the smaller layers.

Functional Erlang	the smallest layer the standard functional language with some additions inherited from Prolog, such as unification instead of binding/equality.
Concurrent Erlang	includes Processes and Messages
Distributed Erlang	includes Remote Processes
Fault-Tolerant Erlang	includes libraries and design patterns from the OTP

Erlang - Resources