The push and pull models above are the two most used ways of accessing or playing your media from other devices.  They are not the only options.  Bear in mind that the Remote App can also remote control the Apple TV; therefore you can use the Remote App to browse the Apple TV's library, which will expose its own Home Sharing connection back to your server, and play your server's media using the 'pull' model.  Alternatively, you can use the Remote App to control the the iTunes library and direct it to route media to your Apple TV via AirPlay (i.e. 'push' model).  Ultimately, you accomplish the same thing, however, they are conceptually different ways of playing your media so it is important to understand the difference to ensure you are using your iTunes network to its best potential.