MongoDB is a document-oriented NoSQL database that stores data in flexible, JSON-like BSON documents, making it one of the most popular databases for modern applications requiring horizontal scalability and schema flexibility. Unlike relational databases, MongoDB allows each document in a collection to have different fields, enabling rapid development and easy adaptation to changing requirements. A critical mental model: MongoDB trades strict schema enforcement for development velocity — you get flexible data modeling and powerful query capabilities, but you must design your schema intentionally around your access patterns rather than relying on database constraints to enforce structure.