Just to repeat what others have already said. NoSQL is basically any data store that is not relational. So it describes data stores that are key/value pair, document, object, graph and column store just to name a few.

Basically a number of these solutions were created to address the scalability issues that companies faced when trying to scale at extreme levels, think Facebook and Twitter. Anyway, it's a really interesting topic, at least to me.

Derek

http://nosqldatabases.com