The need to look at Non SQL systems arises out of scalability issues with relational databases, which are a function of the fact that relational databases were not designed to be distributed (which is key to write scalability), and could thus afford to provide abstractions like ACID transactions and a rich high-level query model. All NoSQL databases try and address the scalability issue in many ways – by being distributed, by providing a simpler data / query model, by relaxing consistency requirements, etc