The seven main parts of the phone (main chassis, battery, display assembly, rear camera, receiver, speaker and cover) can be swapped without changing anything else. The screen can be changed using just your hands, without any screwdrivers or tools.
Fairphone consists of hardware parts that aren't chosen by pure cost/performance evaluation. They take into consideration where the parts are coming from and what social impact in those regions are they making when choosing a particular component.
The phone consists of just 3 hardware modules - "The Spine", "The Hear" and "The Brain". The Spine has an LCD, buttons, speakers and a microphone, The Brain has CPU, GPU, memory and camera, and The Heart has a battery and any extra functionality depending on the module.
With Ara you change hardware parts without needing to power down. Even battery modules can be hot-swapped because the frame itself holds enough power to operate for a small amount of time.
3D Systems make 3D-printing equipment and have partnered with Project Ara to create a continuous 3D printer that can quickly print large amounts of Ara module enclosures. Going forward, it's possible that multiple part of a typical manufacturing process could be delegated to 3D printers.
A $50 version of the phone, that consists only of modules that are required for it to work, bypassing even a cellular connection, instead having an internet connection over Wi-Fi, is planned.