Configuring Red Hat OpenStack Platform for Testing

Hi,
I have tested RHEL 7.1 OSP 8 using PackStack. Since there is limited customization possibility in Packstack configuration, now I am planning on step-by-step configuration. Can anyone explain me on whether to use overcloud/undercloud concept or otherwise to use module-wise installation guide? What is the difference between those two and what are the usecases of those two?