Ember Data is a peristence layer for Ember.Js. Unlike Ember, which currently has a candidate for a 1.0 release, Ember Data is still very much a work in progress. This has been a source of confusion for people who are learning Ember, as the two frameworks are complimentary but currently exist in different realms of stability.
Ember Data has ambitious goals, and it has come a long way in the last year. If you’re the kind of programmer who loves working on upcoming stuff, you might find it exhilarating. On the other hand, it is completely understandable if you’d want to avoid it. Deprecations and changing APIs can be frustrating and time consuming.
One thing that is not always clear to people starting with Ember is that Ember works perfectly well without Ember Data! Trust me on this: Discourse doesn’t use Ember Data for peristence and it’s working quite well. Moreover, using AJAX with Ember is something that is not difficult to do.