BTX is a form factor for motherboards, originally intended to be the replacement for the aging ATX motherboard form factor in late 2004 and early 2005 which was met with mixed reactions.
Because faster components produced more heat it was thought that would require major changes in mid-1990s ATX design. The motherboard manufacturers weren't keen to redesign the ATX boards to accommodate the next generation of hotter-running processors, processors manufactured by the same designers of the BTX technology. There was early support of Gateway, and later by Dell. But today most manufacturers no longer use it.
BTX form factor linied up all the heat-producing components between air intake vents and the power supply's exhaust fan, Intel found that the CPU and other components could be cooled properly by passive heat sinks. A heat sink is a block of aluminum or other metal, with veins throughout, that sits on top of the CPU, drawing its heat away. Fewer fans and a more efficient airflow path create a quieter configuration overall. While the BTX design benefits any modern onboard implementation, Intel's recommitment to lower-power CPUs has lessened the need to move to more expensive BTX systems.