SQL, NoSQL both at high level can be referred to a form of data storage mechanism which difference by the way it holds the data.

SQL's architecture is based on RELATIONAL DATABASES concepts.

NoSQL is based on NON-RELATIONAL DATABASES concepts.

SQL DATABASES:

Structured Query Language (SQL) - data are stored in a predefined schema. For example take an address book where we might have informations like name, phone number, address, city. Likewise in SQL we will define the structure of database before storing information into it. This database can have multiple tables with multiple columns and rows. Tables can be related by reference keys. Each row in table is record/entry whereas each columns represent specific information like name, phone number, address, city.

Structured Query Language (SQL) is a programming language for relational database management and data manipulation. SQL is used to query, insert, update and modify information stored in the database. 

Popular SQL database are Oracle, MySQL, MS SQL Server, Microsoft Azure, MariaDB

NOSQL DATABASES:

NoSQL as name defines is unstructured that can expanded horizontally and vertically any time. Data stored in this database is similar to JSON format which can keep grouping data under a main reference. NoSQL can be useful when application requires dynamic data storage as it provides facility to add new field/data store as and when required. To start storing data, the database can define a minimal schema and can be altered as time goes on and as required.

Data manipulation on NoSQL database are done through object oriented APIs. Common types of NoSQL databases are wide-column, key-value, document, graph.

Popular NoSQL database are MongoDB, HBase, CouchDB, Cassandra