is passed, the operation will be executed immediately with the results passed to the callback.
is not passed, an instance of Query is returned, which provides a special QueryBuilder interface for you.

Let‘s take a look at what happens when passing a callback:

var Person = mongoose.model(‘Person‘, yourSchema); // Query

// find each person with a last name matching ‘Ghost‘, selecting the `name` and `occupation` fields
Person.findOne({ ‘name.last‘: ‘Ghost‘ }, ‘name occupation‘, function (err, person) {
  if (err) return handleError(err);
  console.log(‘%s %s is a %s.‘, person.name.first, person.name.last, person.occupation) // Space Ghost is a talk show host.
})

Here we see that the query was executed immediately and the results passed to our callback. All callbacks in Mongoose use the pattern: callback(error, result). If an error occurs executing the query, the errorparameter will contain an error document, and result will be null. If the query is successful, the errorparameter will be null, and the result will be populated with the results of the query.

Anywhere a callback is passed to a function in Mongoose, the callback follows the patterncallback(error, results).

Now let‘s look at what happens when no callback is passed:

// find each person with a last name matching ‘Ghost‘
var query = Person.findOne({ ‘name.last‘: ‘Ghost‘ });

// selecting the `name` and `occupation` fields
query.select(‘name occupation‘);

// execute the query at a later time
query.exec(function (err, person) {
  if (err) return handleError(err);
  console.log(‘%s %s is a %s.‘, person.name.first, person.name.last, person.occupation) // Space Ghost is a talk show host.
})

An instance of Query was returned which allows us to build up our query. Taking this example further:

Person
.find({ occupation: /host/ })
.where(‘name.last‘).equals(‘Ghost‘)
.where(‘age‘).gt(17).lt(66)
.where(‘likes‘).in([‘vaporizing‘, ‘talking‘])
.limit(10)
.sort(‘-occupation‘)
.select(‘name occupation‘)
.exec(callback);