Relx assembles releases for an Erlang/OTP release. Given a release specification and a list of directories in which to search for OTP applications it will generate a release output. That output depends heavily on what plugins available and what options are defined, but usually it is simple a well configured release directory.
Example Configuration
$ git clone https://github.com/tsloughter/erlangdc2013.git
$ cd erlangdc2013
$ cat relcool.config
{release, {erlangdc, "0.0.1"},
[estatsd
,erlangdc]}.
{extended_start_script, true}.
$ ./relcool
...
$ ls _rel/
bin erts-5.10.1 lib log releases