NLX is a new form factor used in "low profile" motherboard types. They have a unique design where the expansion slots are placed sideways on a special riser card to optimally use the space. . Some manufactures design and manufacture their own motherboard designs, which don't conform to either standard. Basically, the components are laid out differently than an AT or ATX, but it will work as a PC. Such motherboard is known as a proprietary design motherboard. 
The BTX (Balanced Technology Extended) form factor will to be the replacement for the ATX form factor. It was proposed by Intel to address some of the issues that arose from using newer technologies, which demand more power and produce more heat on motherboards compliant with the ATX specification.
