rebar is an Erlang build tool that makes it easy to compile and test Erlang applications, port drivers/executables and releases.
rebar is a self-contained Erlang script, so it's easy to distribute or even embed directly in a project. Where possible, rebar uses standard Erlang/OTP conventions for project structures, thus minimizing the amount of build configuration work. rebar also provides dependency management, enabling application writers to easily re-use common libraries from a variety of locations (git, hg, etc).
You can see a demonstration of converting an existing app to rebar here.