Project Ara is one of the more interesting and ambitious things we've seen come out of Google in the mobile arena in some time. The basic idea is a smartphone-like device, but one that has swappable components that can be changed out simply to either improve the experience of the device or add new features. Cameras, batteries and sensors can be added at will to a Project Ara phone without changing how the device works otherwise.
Right now, things are still very early for Project Ara. Google has begun a consumer trial of the phones and components in Puerto Rico, but beyond that is still recruiting manufacturers to make new modules and getting a feel for how consumers will react to the idea of an easily-upgradeable phone. There's a target "entry" price of $50 for Project Ara devices, which is pretty ambitious.
We're not sure if Project Ara will end up taking the world by storm, but we are sure that it's something we will be keeping an eye on.