The Exasol DWH is a parallel RDBMS running on a cluster of Intel server nodes. The data is stored in shared-nothing manner but with redundant copies on other nodes, so no data is lost if some nodes disappear.
The high performance is achieved by using much RAM (think of 20 or more servers with 32 GB...), working with compressed data and doing as much as possible in parallel with local data. On a well sized system the query optimizer can use in-memory algorithms instead of disk centric algorithms which gives a big boost to full-table scans, sorts, complex joins. RAM access is hundreds of times faster than disk access.
The system is designed for complex queries on vast amounts of data, e.g. ROLAP, Business Intelligence and so on. For fast inserts or updates a conventional dbms would be the better choice.
On their homepage (www.exasol.com) there is an english whitepaper available for download after registration.

I've had several discussions with Stu Greenberg, their VP of Business Development and the guy leading the charge to bring Exasol to the US. I've summarized in my blog:

http://360degreeinsight.com/2008/04/23/exasol-ag/

Rick Abbott