MongoDB is a No-SQL database that stores data in JSON format. The data is organised into documents and these documents are stored in collections, which in turn are located in the database. The data is denormalised which is in contrast to the normalised data structure of an SQL database.
An analogy with an SQL database is that a collection is a table and a document is a record in that table.
The MongoDB website provides a brief introduction to MongoDB and some key concepts behind the No-SQL document structure. If you are unfamiliar with this database type you must read this first.