http://en.wikipedia.org/wiki/Cassandra_(database) http://en.wikipedia.org/wiki/Cassandra_(database) has a pretty good overview if you're not familiar with Cassa

http://en.wikipedia.org/wiki/Cassandra_(database) http://en.wikipedia.org/wiki/Cassandra_(database) has a pretty good overview if you're not familiar with Cassandra. Cassandra only guarantees eventual consistency where as most RDBMS systems are focused on always showing a consistent view of data. Because of this (and other trade-offs), Cassandra is able to provide much higher IO rates as well as improved fault tolerance and linear scalability. Twitter, Digg, and Facebook all happen to have usages which fit perfectly with this model. If you see a status update a few seconds (or even minutes) late occasionally it's no big deal. If your bank funds aren't matching for that long, it can be a big deal. Edit: I just started looking into Cassandra a few days ago so someone please correct me if I've mis-stated anything.