HBase is the Hadoop database. Think of it as a distributed, scalable, big data store.
Use HBase when you need random, realtime read/write access to your Big Data. This project's goal is the hosting of very large tables -- billions of rows X millions of columns -- atop clusters of commodity hardware. HBase is an open-source, distributed, versioned, column-oriented store modeled after Google's Bigtable: A Distributed Storage System for Structured Data by Chang et al. Just as Bigtable leverages the distributed data storage provided by the Google File System, HBase provides Bigtable-like capabilities on top of Hadoop and HDFS.
See the Architecture Overview, the Apache HBase Reference Guide FAQ, and the other documentation links on the left!
June 15th, 2012 Birds-of-a-feather in San Jose, day after Hadoop Summit
May 23rd, 2012 HackConAthon in Palo Alto
May 22nd, 2012 HBaseCon2012 in San Francisco