Device Tree is a data structure for describing hardware.

Rather than hard coding every detail of a device into an operating system, many aspects of the hardware can be described in a data structure that is passed to the operating system at boot time.

In the past kernel modules were compiled into the kernel, but this is rather restrictive.

Initially (Wheezy) used the traditional approach, but hardware support has been progressively moved to Device Tree. It is certainly NOT Pi specific.

An OS can supply many Loadable Kernel Modules to provide support for a wide range of hardware which are loaded on demand.
This can be done explicitly (configured by user), but Device Tree provides a mechanism to load at boot time, and usually configures hardware to match (the Pi peripherals allow most GPIO pins to be configured in any of 7 different modes).

Device Tree is a data structure for describing hardware.

Rather than hard coding every detail of a device into an operating system, many aspects of the hardware can be described in a data structure that is passed to the operating system at boot time.

In the past kernel modules were compiled into the kernel, but this is rather restrictive.

Initially (Wheezy) used the traditional approach, but hardware support has been progressively moved to Device Tree. It is certainly NOT Pi specific.

An OS can supply many Loadable Kernel Modules to provide support for a wide range of hardware which are loaded on demand.
This can be done explicitly (configured by user), but Device Tree provides a mechanism to load at boot time, and usually configures hardware to match (the Pi peripherals allow most GPIO pins to be configured in any of 8 different modes).