MongoDB is a document database made by MongoDB, Inc. A record is a document, a set of field and value pairs similar to a JSON object, and values can contain other documents and arrays. Documents are stored in collections, which are like tables in a relational database but do not enforce a fixed schema, so the data model can change without downtime.
Because a document can mirror an object in the application code, no object-relational mapping layer is needed, and embedded documents reduce the need for joins. MongoDB supports multi-document ACID transactions (all-or-nothing groups of reads and writes), replication with automatic failover, and sharding, which spreads data across several servers for horizontal scaling. It also provides aggregation, full-text search, vector search for AI applications, geospatial search and time series data.
MongoDB runs as MongoDB Atlas, a fully managed cloud service across clouds, or self-managed with the free Community Edition or the paid Enterprise Advanced. Atlas has a free cluster and is otherwise billed per hour by cluster type and size.