From the project page:
Google's Bigtable, a distributed storage system for structured data, is a very effective mechanism for storing very large amounts of data in a distributed environment.Note that two of the four early contributors -- Jim Kellerman and Michael Stack -- are from Powerset. Some members of the Powerset team also have been trying to run Hadoop on Amazon EC2.
Just as Bigtable leverages the distributed data storage provided by the Google File System, Hbase will provide Bigtable-like capabilities on top of Hadoop.
Data is organized into tables, rows and columns, but a query language like SQL is not supported. Instead, an Iterator-like interface is available for scanning through a row range (and of course there is an ability to retrieve a column value for a specific key).
See also my earlier posts, "Google Bigtable paper" and "Google's Bigtable".
See also my earlier posts, "Yahoo building a Google FS clone?" and "GFS, MapReduce, and Hadoop".
Update: Looking at the HBase source tree, there appear to be other contributors who are not listed on the project page. Not surprising to see that Mike Cafarella and Doug Cutting were deeply involved.