MongoDB is an open-source document database and leading NoSQL database. MongoDB is written in c++. MongoDB is a cross-platform, document-oriented database that provides, high performance, high availability, and easy scalability. MongoDB works on the concept of collection and document.
The database is a physical container for collections. Each database gets its own set of files on the file system. A single MongoDB server typically has multiple databases. It is the equivalent of an RDBMS table. A collection exists within a single database. Collections do not enforce a schema. Documents within a collection can have different fields. Typically, all documents in a collection are of similar or related purpose and the document is a set of key-value pairs and has dynamic schema. Dynamic schema means that documents in the same collection do not need to have the same set of fields or structure, and common fields in a collection’s documents may hold different types of data.
Currently, MongoDB is the most famous DB and been chosen as an alternative to SQL database so you can see how to use Mongo DB with .Net using C#: