Typically, NoSQL database platforms have a distributed, fault-tolerant architecture with the ability to store and access vast amounts of unstructured data. Most useful NoSQL systems employ a distributed architecture, with the data stored redundantly across multiple servers. Scale out is achieved simply by adding commodity servers 'hot' (to the live database).
A number of nosql technologies are competing for this 'space', each more or less suitable for a particular task. Nosql offerings currently include (in no special order) Couchbase, MongoDB, Cassandra, djondb, eXtreme Scale, FoundationDB, HBase, OrientDB, Riak (a key pair store), etc).
They are not always the solution but come into their own when performance and concurrency is more important than consistency. In many cases a MySQL cluster will provide equivalent performance and a more broadly supported (SQL) interface and environment.
We are most familiar with MongoDB which is enjoying rapid growth (both in terms of maturity and acceptance) especially in the financial sector.
If you are contemplating the development of a scaleable web application that may benefit from a nosql database development, please give us a call or drop us a line.