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Here is a snip from the Apache Hadoop project website on what it is:
The Apache™ Hadoop™ project develops open-source software for reliable, scalable, distributed computing.
The Apache Hadoop software library is a framework that allows for the distributed processing of large data sets across clusters of computers using a simple programming model. It is designed to scale up from single servers to thousands of machines, each offering local computation and storage. Rather than rely on hardware to deliver high-avaiability, the library itself is designed to detect and handle failures at the application layer, so delivering a highly-availabile service on top of a cluster of computers, each of which may be prone to failures.
To learn more about the Apache Hadoop project, see here: http://hadoop.apache.org/
For more information on Hadoop for Windows Azure, see here: https://www.hadooponazure.com/ and to register to be considered for this CTP, please see here: https://connect.microsoft.com/SQLServer/Survey/Survey.aspx?SurveyID=13697