This document provides an introduction to MongoDB, a scalable and high-performance document-oriented database. It discusses how MongoDB stores data as documents rather than in tables, as relational databases do, and how this makes it a good fit for storing application objects and JSON data. The document also explains some key differences between MongoDB's structure of databases, collections, and documents and the table-based model of relational databases. It provides examples of how embedded and referenced data relationships can be modeled in MongoDB. Finally, it introduces Mongoid and MongoMapper as Ruby libraries for interacting with MongoDB databases.