MongoDB, also known as Mongo, is an open-source document database used in many modern web applications. It is classified as a NoSQL database because it does not rely on a traditional table-based relational database structure.
Instead, Mongo uses JSON-like documents with dynamic schemas. This means that, unlike relational databases, MongoDB does not require a predefined schema before you add data to a database. You can alter the schema at any time and as often as is necessary without having to set up a new database with an updated schema.