This document introduces NoSQL databases. It discusses how SQL databases struggled to scale with the growth of internet companies like Facebook and Twitter in the 2000s. This led to the development of non-relational NoSQL databases like Google's BigTable that could scale horizontally across many servers. The document defines NoSQL databases as non-relational, open source, schema-less and cluster friendly. It outlines the four main data models used in NoSQL - key-value, document, column family and graph databases. In conclusion, it emphasizes that NoSQL provides an alternative solution for applications that need horizontal scalability.