Thanks @Sanjeev , very useful information indeed.
If I may make a suggestion to the HortonWorks (and Hadoop as a general) community, as a real newbie : the HDP Sandbox tutorials are a really GREAT way to understand the main concepts of Hadoop (HDFS and MapReduce), as well as other tools (HCatalog, Hive, Pig, …).
BUT, I think there is a real huge gap between these tutorials and the moment when you are actually developing actual code for Hadoop.
I found only one usful (to me, as a rookie : very detailed and up to date) resource : https://github.com/hortonworks/hadoop-tutorials/blob/master/Community/T09_Write_And_Run_Your_Own_MapReduce_Java_Program_Poll_Result_Analysis.md
If you have any other such pointers, please share!