Explain MongoDB's data hierarchy: Database, Collection, Document.

MongoDB organizes data in three levels: a database holds collections, and a collection holds documents.

• A database is the top-level container for collections. MongoDB comes with the admin, local and config databases. By default, all databases keep their files in one data directory, and the directoryPerDB setting gives each database its own directory.
• A collection is a group of documents, like a table in a relational database. Its documents can have different schemas. MongoDB creates a collection when you insert its first document.
• A document is one record, a JSON-like structure stored as BSON, a binary form of JSON. It can hold nested documents and arrays, and it can be at most 16MB.

The diagram below shows an example structure.

Database: ecommerce
├── Collection: users
│   ├── Document: {_id: 1, name: "John", email: "john@email.com"}
│   └── Document: {_id: 2, name: "Jane", age: 25, city: "NYC"}
└── Collection: products
    └── Document: {_id: 1, title: "Laptop", price: 999.99}