A History of Erlang and the accompanying Presentation Slides by Joe Armstrong are a must read for anyone interested in PL history.
Erlang was designed for writing concurrent programs that "run forever". Erlang uses concurrent processes to structure the program. These processes have no shared memory and communicate by asynchronous message passing. Erlang processes are lightweight and belong to the language and not the operating system. Erlang has mechanisms to allow programs to change code "on the fly" so that programs can evolve and change as they run. These mechanisms simplify the construction of software for implementing non-stop systems.
(Link to previous HOPL-III papers on LtU).