Basically, its essentially the same as ATX, except that the board can be up to 12"x13" in size. These boards are uncommon, and the Extended ATX form factor is likely to receive significant competition from the new WTX form factor, which was designed especially for workstations. The larger motherboard size means that special Extended ATX cases are generally required.
extended atx was mainly created for users who wanted four pci-xpress slots as its very hard to fit it on an atx board. but yer they support most sockets and cpus but there arent to many around. Google will be the way forward to find ones that your looking for!!!