The most convenient way to get RAP is the Eclipse package for RCP and RAP Developers from the Indigo download page. This IDE includes everything you need to develop RCP and RAP applications.
The RAP Tooling contains useful tools, templates and documentation for RAP developers. It also installs the RAP target platform which is included. Learn how to install RAP and build your first RAP application.
This contains the RAP runtime, including sources and a simple demo application. Note that this is already included in the RAP Tooling.
To develop RAP applications you need to set the RAP runtime as your target platform. Learn how to work with the target platform.
This is a target platform, do not try to install it into the Eclipse IDE!