HDP is a hadoop distribution stack like what other vendors do (ie cloudera, mapr, etc).
HDP is the only 100% true open source
to learn how to use all the HDP services (ie hadoop, pig, hive, etc), hortonworks engineers have assembled a sandbox. You can download it and play.
Then when you are ready to install software on a cluster, you would use the HDP official distribution.
HDP Sandbox = play ground for your first visit into hadoop
HPD = full cluster software