HCatalog makes Hive metadata available to users of other Hadoop tools like Pig, MapReduce and Hive. It provides connectors for MapReduce and Pig so that users of those tools can read data from and write data to Hive’s warehouse.

HCatalog’s table abstraction presents users with a relational view of data in the (HDFS) and ensures that users need not worry about where or in what format their data is stored, so users don’t need to know if data is stored in RCFile format, text files, or sequence files.

It also provides a notification service so that workflow tools, such as Oozie, can be notified when new data becomes available in the warehouse.

HCatalog provides HCatInputFormat/HCatOutputFormat to enable MapReduce users to read/write data in Hive’s data warehouse. It allows users to read only the partitions of tables and columns that they need. And it provides the records in a convenient list format so that users do not need to parse them.

more via MapReduce On Hive Tables Using HCatalog | Javalobby.