I have heavily worked on the MongoDB NoSQL Database and Oracle.
SQL database has its own predefined schema to store structured data.
In NoSQL database, there is no predefined schema, here schema is most dynamic element based on the data elements.
SQL Databases are vertically scalable, which means if we want to scale SQL base database, we need to give hardware boost on which the DBMS System is installed. This is where it sometimes goes for the limitation of scalability.
NoSQL databases are horizontally scalable, means if we want to scale it, we need to add more nodes and create distribution network based on our own need and required power. This is how they reduce load on the database
In SQL based databases, to define and manipulate data we can use SQL (Structured Query Language), which is very powerful nowadays.
In terms of NoSQL database, queries focus on collection and documents. Sometimes it is called UnQL (Unstructured Query Language). This is still in the evolution phase, so it varies from vendor to vendor of the NoSQL database.
For more on key differences, my blog: Difference between SQL and NoSQL database