What is MongoDB?

From Wikipedia

MongoDB (from "humongous") is a cross-platform document-oriented database. Classified as a NoSQL database, MongoDB eschews the traditional table-based relational database structure in favor of JSON-like documents with dynamic schemas (MongoDB calls the format BSON), making the integration of data in certain types of applications easier and faster

From Rackspace blog

MongoDB is the de facto choice for NoSQL applications. Our customers want MongoDB. It is the No. 1 requested previously unsupported database. MongoDB is seeing widespread adoption by developers because it is super easy get started and to use, and it boosts productivity. Further, MongoDB has removed the schema barrier, which allows developers to focus on building applications, not databases. It also has widespread support for every language we see being relevant to the future of the web – C, C++, C#, JavaScript, Node.js, Objective-C and Scala, just to name a few. These reasons drive MongoDB adoption today and have pushed MongoDB to quickly become the database of choice for non-relational applications. MongoDB is here to stay.

More details on mongodb.org