NoSQL is a blanket term for the databases that solve the scalability issues that are common among relational databases. This term, in its modern meaning, was first coined by Eric Evans (NoSQL naming available at http://blog.sym-link.com/2009/10/30/nosql_whats_in_a_name.html). It should not be confused with the database named NoSQL (NoSQL: the database available at http://www.strozzi.it/cgi-bin/CSA/tw7/I/en_US/nosql/Home%20Page).
The NoSQL solutions provide scalability and high availability, but may not guarantee ACID: atomicity, consistency, isolation, and durability in transactions. Many of the NoSQL solutions including Cassandra sit on the other extreme of ACID, named BASE (Basically Available, Soft-state, Eventual consistency