MongoDB is a document-oriented database, not a relational one. Non relational means it does not store data in tables but in the form of JSON document. Here in MongoDB row is replaced with document and table is replaced with collection. Collection can be considered as a group of documents.
Document is the basic unit of data for MongoDB, roughly equivalent to a row. It is a data structure composed of field and value pairs. MongoDB documents are similar to JSON objects. The values maybe documents, arrays, and arrays of documents.