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.
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.
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