... to describe non-relational databases. There are many different ways to store data in various database designs. They are mainly of the following: Relational (mysql, postgresql), Column based (HBase, Cassandra), Document based (Mongodb, CouchBase), Hash based (redis, memcached), and Graph based (neo4j). Databases can also be categorized as single system databases or distributed databases. Mongodb and most (all?) relational dbs are single system databases. HBase, Cassandra, CouchBase, Riak are distributed databases.
NoSQL is developed to solve problems that traditional SQL databases are unable or difficult to solve, such as real time updates on a massive scale, or being able to scale horizontally with relative ease, or lighting fast query speeds. They also have drawbacks such as eventual consistency, and synchronization issues.
A good architect / programmer should have a solid analysis of the usage case against different flavors of DBs and perform benchmarks, be familiar with their fail over / recovery procedures, and have a good understand of the underlying technologies before considering using them in a serious production environment.