Hortonworks is a massive contributor to the open-source Hadoop community focused on building it into a broadly capable data-management platform. Hortonworks sets itself apart from competitors Cloudera and MapR by eschewing proprietary components. Everything in the Hortonworks Data Platform (HDP) is freely available as open-source software.
To its critics -- the aforementioned competitors -- Hortonworks pushes this open-source point to a fault, waiting to ship sought-after functionality until it is community sanctified and avoiding new (and perhaps technically better approaches) that are not entirely open source. Hortonworks is sticking with -- and trying to improve -- Hive, for example, while Cloudera promises better SQL-on-Hadoop performance with Impala, which is open source, technically, but best managed with proprietary Cloudera Manager software.
In short, HDP is the conservative Hadoop distribution, and Hortonworks reportedly undercuts its competitors on support costs. Hortonworks makes the point that theres no threat of vendor lock-in with its distribution, and everything shipping has been thoroughly tested and proven. You wont get any surprises, but nor will you get anything ahead of the rest of the community in performance, ease of management, or functionality.