NoSQL is a class of database management systems (DBMS) that do not follow all of the rules of a relational DBMS and cannot use traditional SQL to query data. The term is somewhat misleading when interpreted as "No SQL," and most translate it as "Not Only SQL," as this type of database is not generally a replacement but, rather, a complementary addition to RDBMSs and SQL.
NoSQL-based systems are typically used in very large databases, which are particularly prone to performance problems caused by the limitations of SQL and the relational model of databases. Many think of NoSQL as the modern database of choice that scales with Web requirements. Some notable implementations of NoSQL are Facebook's Cassandra database, Google's BigTable and Amazon's SimpleDB and Dynamo.