1) HDP is a stack that is maintained by Hortonworks. It is a collection of services and versions of the services certified by Hortonworks to work together as a hadoop system. With a version of HDP "stack", you will have a recommended set of versions of services installed.
You can see the growth of the HDP stack in the diagram titled "Ongoing innovation in Apache", here :
2) To use HDP repo you don't need an enterprise license. HDP is completely open source
3) Before starting off things in a production system, you may want to check install using sandbox and get familiar with HDP:
and then go ahead and look at :
To get a starting point into HDP docs, look at :
http://docs.hortonworks.com/index.html -- This has docs for every version of HDP and ambari