MongoDB is a very popular document-oriented NoSQL database. MongoDB provides a high-performance engine for storage and query retrieval. In a document-oriented database, we store the data into collections of documents, in this case JSON-like documents called BSON (Binary JSON), which provide us with a dynamic-schema data structure. MongoDB implements functionalities such as ad hoc queries , replication , load balancing , aggregation , and Map-Reduce . MongoDB is perfect for an operational database. However, its capabilities as a transactional datasource are limited. We can see the similarities between the structures of a relational database (RDBMS) and MongoDB in the following diagram. You can find more information about MongoDB from its official website, http://www.mongodb.org/.
For complete reference about SQL databases, please refer to http://www.w3schools.com/sql/sql_quickref.asp/.