A document database is a database that can store and retrieve a document (generally a json document) at once.
The data unit is generally not the record as in a relational database but the document.
You create/update/overwrite documents in a collection.
The document model works well with use cases such as:
where each document is unique and evolves over time.
Most of the actual database can store document but the first one to support it were nosql database such as: