For the last 50 years there has been one and only one way of designing a database. SQL Database management systems (DBMS) have been the uncontested standard. (A better name is ‘relational’ because SQL is just the name of the language used to access them, but we will stick with it.)
But things have changed in the last 10-15 years. Databases that disobey relational rules are coming into vogue. The term NOSQL is used for these non-relational databases (the above chart is work in progress, but I have been looking at these systems.)
What is the difference between classic databases and this new breed?
Bob Browning's blog 