The MongoDB is now most popular cross platform NoSQL open source database management system. The MongoDB is written using the C++ language and developed by 10gen but now it's called MongoDB Inc.
The MongoDB is document oriented database and its store the data in document with JSON format. i.e.
UserName: 'Anil Singh',
WorkingArea: 'IT Software',
Comments: 'put anything else...'
In the JSON document, Table name called collection, row called document, column name called Field, Joins called linking etc.
Why MongoDB is different from SQL Server?
The MongoDB store the data in documents with JSON format but SQL store the data in Table format.
The MongoDB provides high performance, high availability, easy scalability etc. rather than SQL Server.
In the MongoDB, we can change the structure simply by adding, removing column from the existing documents.