Callbacks and Error Handling in Node.js

So, we have managed to open a connection to MongoDB and run some simple queries, but there were probably a couple of elements of the code that seemed unfamiliar; for example, the syntax here:

.each(function(err, doc) {
        if(doc) {
            console.log('Current doc');
            console.log(doc);
        } else {
            client.close(); // Close our connection.
            return false;   // End the each loop.
        }
     });

This is what is known...