The push notifications feature does not depend on other features of Telerik Backend Services and could be used individually (in the context of the subscription tier for Backend Services). The benefit of relying on them is the out-of-the-box seamless client-side integration with the platforms supported by AppBuilder (WP coming very soon). For the means of targeting various apps/users, additional parameters could be assigned from the client side app to the registered devices and used to target push notifications to a tailored segment. The fully elaborated REST API for creating, targeting and sending push notifications can be reached by basically any programming language or tool that can make a HTTP request.
On the other hand, the AppBuilder team published a very informative blog post
on how to create your very own push notifications server (covers Anrdoid and iOS platforms) and make use of the Cordova PushPlugin in your hybrid IDE, in order to obtain such functionality. The community forums also offer a handful of resources on this.