This document provides an introduction to MongoDB, a popular document-oriented database. It discusses how MongoDB stores data in flexible, JSON-like documents rather than rigid tables. It also covers MongoDB's features like replication, sharding, indexing, querying, map-reduce functions and how it provides a scalable and flexible alternative to traditional relational databases. The document also discusses some of the theoretical underpinnings of non-relational databases like MongoDB, including the CAP theorem and ACID versus BASE models of data consistency.