NoSQL is an upcoming category of Database Management Systems. Its main characteristic is its non-adherence to Relational Database Concepts. NoSQL means "Not only SQL". The concept of NoSQL databases grew with internet giants such as Google, Facebook, Amazon etc. who deal with gigantic volumes of data.
When you use a relational database for massive volumes of data, the system starts getting slow in terms of response time. To overcome this, we could "scale up" our systems by upgrading our existing hardware. The alternative to the above problem would be to distribute our database load on multiple hosts as the load increases. This is known as "scaling out".
NoSQL database is non-relational databases that scale-out better than relational databases and are designed with web applications in mind. They do not use SQL to query the data and do not follow strict schemas like relational models. With NoSQL, ACID (Atomicity, Consistency, Isolation, Durability) features are not guaranteed always.