Many people in the Apache Hadoop community have asked Yahoo! to publish the version of Apache Hadoop they test and deploy across their large Hadoop clusters. As a service to the Hadoop community, Yahoo is releasing the Yahoo! Distribution of Hadoop -- a source code distribution that is based entirely on code found in the Apache Hadoop project.
This source distribution includes code patches that they have added to improve the stability and performance of their clusters. In all cases, these patches have already been contributed back to Apache, but they may not yet be available in an Apache release of Hadoop.
Read more and get the Hadoop distribution from Yahoo