This combines the advantages of Lucene - such as full-text search, ad-hoc queries and complex business intelligence reporting - with the advantages of using a full-featured distributed database.
Update: Cloudant has added some more technical details.
BigCouch is based on CouchDB but adds ideas from Amazon's Dynamo to provide better scalability. Cloudant also added a number of baked-in analytics capabilities to BigCouch. It's available as a public cloud service or an on-premise, private cloud solution. Cloudant Search will only be available with the paid versions of BigCouch and not with the free open source version.
Those looking for an open source Lucene integration with CouchDB can check out couchdb-lucene.
Cloudant was founded in 2008 by three particle physicists at MIT in order to address common issues scientists faced when working with multi-petabyte data sets. The company received seed funding from Y Combinator and later closed $1 million in series A funding from other investors.