MongoDB is a document database that stores data in JSON-like documents in a flexible and scalable way, meaning it has no fixed table structure. It is a NoSQL database and it can be used as both the backend for websites, mobile apps, and other applications as well as the frontend for JavaScript applications. It is designed to be fast and efficient for handling large amounts of data and provides high availability with robust fault tolerance. It is written in C++ and has bindings to numerous programming languages such as Java, Python, Objective-C, Ruby, Go, and NodeJS. The data model of MongoDB enables developers to represent hierarchical structures such as trees or graphs in a natural way.
It has been used to build many popular web applications including Twitter, LinkedIn, eBay, and Craigslist.