What is a device tree blob? Why is it needed?

Since the time when a number of ARM sub-architectures are entering the ground, it's been a big headache to the maintainers of the Linux source as there were huge number of duplicates to support specific sub-architecture and board configurations making the Linux Mainline so big and clumsy. To avoid this, the team came up with an idea to keep the board specific files away from the Kernel mainline and thus the birth of Device Tree Source (.dts) files which will be compiled as separate binary, Device Tree Blob (.dtb). With device tree, it is possible to use a single kernel image on different boards using the same SoC by simply passing a different device tree containing board specific data structure.. When booting, it is the boot-loader which passes this device tree to the kernel. At present, it is made mandatory to have Device Tree. More story about device tree can be found at xillybus.com, elinux.org, freebsd.org and more. Also check this document.