MongoDB is an open-source document database built on a horizontal scale-out architecture that uses a flexible schema for storing data. MongoDB has a worldwide following in the developer community and is one of the most popular NoSQL databases.
How it works
Instead of storing data in tables of rows or columns like SQL databases, each record in a MongoDB database is a document described in BSON, a binary representation of the data. Applications can then retrieve this information in a JSON format.
Document databases are highly flexible, allowing variations in the structure of documents and storing documents that are partially complete. One document can have others embedded in it. Fields in a document play the role of columns in a SQL database, and like columns, they can be indexed to increase search performance.
MongoDB was built on a scale-out architecture, a structure that allows many small machines to work together to create fast systems and handle huge amounts of data.
MongoDB is free to use.