A promise can either be fulfilled or it can be rejected.

var promise = fetchTheAnswer();

promise.then(fulfill, reject);

function fulfill(answer) {
  console.log("The answer is " + answer);
}

function reject(reason) {
  console.log("Broken promise! Reason: " + reason);
}

You can even chain promises together with .then() and run through a whole slew of asynchronous actions.

var promise = Ember.$.getJSON('/promise-me.json');

promise.then(doThis)
       .then(doThat)
       .then(doOtherStuff)
       .then(handleFulfill, handleReject);

A promise is a method with asynchronous behavior. It returns a "thennable" which is an object that reflects fulfillment (success) or unfulfillment (failure).

You attach handlers for these states using the then method. Each then returns yet another promise.

More about asynchronous routing.