NoSQL database or Post Relational databases are umbrella terms for a loosely defined class of non-relational data stores that break with a long history of relational databases and ACID guarantees. Data stores that fall under this term may not require fixed table schemas, and usually avoid join operations. The term was first popularised in early 2009.
Trends in computer architectures are pressing databases in a direction that requires horizontal scalability. NoSQL-style data stores attempt to address this requirement. Prominent closed-source examples are Google's BigTable and Amazon's Dynamo. Several open-source variants exist including Facebook's Cassandra, Apache HBase, LinkedIn's Project Voldemort and many others.
