The Device Tree (DT), and Device Tree Overlay are a way to describe hardware in a system. An example of this would be to describe how the UART interfaces with the system, which pins, how they should be muxed, the device to enable, and which driver to use.
This guide was first published on Jul 29, 2013. It was last
updated on Oct 19, 2018.
This page (Overview) was last updated on May 04, 2015.
The original BeagleBone didn't use the DT, but the recently released BeagleBone Black was released with the DT and is now using the 3.8 Linux Kernel.
The following pages will attempt to break down the concepts, and give examples on how and why you'd want to use the device tree in your every day development and hacking.