NoSQL databases differ from RDBMS technology in four main areas:

1. Data Model
2. Data Structure
3. Scaling
4. Usage Cost

Data Model RDBMS is traditional way storing the structured data in relational way. Without schema we cannot build/connect to an application. NoSQL is latest technology which is designed to handle the BigData. We can build an application without having to define the schema.
Data Structure Used to store the data which is structured and defined by relations. Designed to handle the unstructured data ( Ex: Text, Email , Image, Video ).
Scaling Required single server to host the entire database and Vertical scaling is costly to afford. This is much cheaper to scale compared to relational database.
Usage Cost Licensed and costly. Open-source and ready to use.