So, what is NoSQL?

• Key-value based database: This is perhaps the simplest kind of database. We are storing data in the form of key-value combinations. Think of a hashmap kind of structure, where we are adding a unique identifier as a key and data objects as values. It is very easy to scale, as long as we have unique keys. Examples of this kind of database include Redis and Riak.
• Column-based databases: We have been using row-based relational databases for a long time. In row-based databases, we think of a record as a single object that can be stored in a database's table row.Examples of databases using a column-based storage mechanism are Cassandra and Vertica.
• Document-based databases: This can be thought of as an extension to key-value based storage. In a key-value-based system, a value can be anything, but a document-based database adds a restriction for proper formatting to be followed for data being stored. The data that is stored as documents. Metadata is provided for each document being stored, for better indexing and searching. Examples of document-based storage databases are MongoDB and CouchDB.
• Graph-based databases: This kind of database is useful when our data records are connected to other records in some way and we need a method to parse this connectivity. A simple example is when we are storing information for people, and we need to capture friendship information, such as P1 is a friend of P2, who in turn is a friend of P4 and P6, so we can capture some relationship between P1, P2, P4, P6, and so on. Examples of databases that use graph-based storage are Neo4J and OrientDB.