BTX, or Balanced Technology eXtended, is the proposed successor to the ATX form factor, the most common form factor for new PCs. It is under development by Intel.
The principal motivation for a new form factor is the excessive amount of heat generated by modern components, especially the CPU. The BTX form factor is designed to improve airflow. Except for the ability to use ATX power supplies, BTX is incompatible with the older standard.
Expansion slots (PCI and PCI Express) have switched sides. One express slot is next to the CPU, allowing it to use some of the cooling power from the fan next to the CPU. The CPU is on the front edge. There is a duct around the CPU, fan, and heat sink. The graphics card will be mounted on a horizontal riser, although it may also be mounted vertically if desired.
BTX will be available in three flavors - picoBTX, microBTX, and BTX. micro is expected to be the most popular. They support two, four, and seven expansion slots, respectively.
Graphics will be handled by one of the PCI Express slots, probably the horizontal one in most cases. ATI released a number of PCI Express motherboards which support PCI-E graphics card around the time BTX became available.
BTX is set to be available in 2005 or 2006, at least as far as mainstream use goes.