MongoDB − It is a document database, that stores data in flexible, JSON-like documents.
Express − It is web application framework for Nodejs.
For more information on these, you can refer the overview chapter. The below diagram depicts architecture of MEAN stack application.
As shown in the above image, we have AngularJS as client side language which processes the request of a client.
Whenever a user makes a request, it is first processed by AngularJS.
Next, request enters second stage, where we have Node.js as server side language and ExpressJS as backend web framework.
Node.js handles the client/server requests and ExpressJS makes request to the database.
In the last stage, MongoDB (database) retrieves the data and sends the response to ExpressJS.
ExpressJS returns the response to Nodejs and in turn to AngularJS and then displays the response to user.