title: What does Express.js do in the MEAN stack?
body: <p>I have recently have gotten into AngularJS and I love it. For an upcoming project I am looking to use the MEAN stack (MongoDB, Express, Angular, Node). I'm pretty familiar with Angular and I have a modest understanding of the purposes of MongoDB and Node in the stack. However, I don't really understand what the purpose of Express.js is. Is it essential to the MEAN stack? What would you compare it to in a traditional MySQL, PHP, javascript app? What does it do that the other three components can't do?</p>
  
  <p>Also, if someone wants to give their own take on how the four parts of the stack work together, that'd be great.</p>