MongoDB is a popular NoSQL database management system that stores data in a document-oriented format. It is designed to handle large amounts of data and provide high performance and scalability. Unlike traditional relational databases, MongoDB does not use tables, rows, and columns to store data, but instead uses collections and documents. This allows for more flexibility in data storage and retrieval, making it a popular choice for modern web applications. MongoDB is open-source and widely used by companies of all sizes for its ease of use, flexibility, and scalability.