As stated on its official website, MongoDB is an open source document database with the scalability and flexibility that you want and the querying and indexing that you need.

MongoDB uses collections to persist a set of JSON documents, and the schema for the documents can change over time without affecting the other documents stored in the collection. The schema-less feature is great when you are working on applications that have different kind of roles and users. A user might utilize some fields, while other users might require some other fields but without the need to fill the unused fields with null values. Instead, fields that are not required are not persisted in the JSON document.