HCatalog, also called HCat, is an interesting Apache project. It has the unique distinction of being one of the few Apache projects that were once a part of another project, became its own project, and then again returned to the original project Apache Hive.
HCat itself is described in the documentation as “a table and storage management layer” for Hadoop. In short, HCat provides an abstraction layer for accessing data in Hive from a variety of programming languages. It exposes data stored in the Hive metastore to additional languages other than HQL. Classically, this has included Pig and MapReduce. When Spark burst onto the big data scene, it allowed access to HCat.
Given HDInsight’s predilection toward WebHCat over WebHDFS, this does seem like a good thing to learn.