MongoDB is an open-source, NoSQL database designed for handling large volumes of unstructured or semi-structured data. Unlike traditional relational databases, MongoDB stores data in flexible, JSON-like documents (BSON), allowing for easy storage and retrieval of complex data types. It is highly scalable, offering horizontal scaling through sharding, and is ideal for applications that require rapid development, high availability, and the ability to handle big data. MongoDB is commonly used in web applications, real-time analytics, and projects where data flexibility and scalability are crucial.