While there are a number of NoSql databases out in the market currently, MongoDB is the leading Open-Source, NoSql database available. MongoDb has a flexible schema, which means that the collections are not bound in a fixed document. The collections in MongoDB are stored in documents called BSON or Binary JSON. The structure of a BSON document is same as a JSON document, which means that every collection in Mongo is enclosed in curly brackets just like a JSON document. When saying Mongo has a flexible schema it means that when a collection has, for example 4 fields, data with number of fields other than 4 can be added to the same collection. The collections are not schema bounds like relational databases.