Have a look at this video of Brian Aker's great 10 minute lightning talk about NoSQL. NoSQL is a database movement which promotes non-relational data stores that do not need a fixed schema.

With the exception of Memcached, the Drupal world is very SQL driven — and for all the right reasons. However, both at Mollom and Acquia we have to deal with some big data volumes and have been using some of the NoSQL technologies like Hadoop, Thrift, Hbase, Tokio Cabinet, etc.

While these are great tools, at the end of the day, NoSQL is an "and" and not a "versus". Plus, I expect the gap to close as there are a couple of interesting projects under way that bring some of the NoSQL advantages to the SQL world. One of them is Brian Aker's own Drizzle project, another project I can't talk about yet ... Sufficient to say, don't ditch your relation database just yet.

Either way, have a look Brian's NoSQL presentation. It's funny!