More and more motherboards now feature a very useful on-board diagnostic LED display.  This component is not essential to the functioning of a PC, but is worth mentioning because of its usefulness in troubleshooting general hardware boot-up problems.  Some boards allow the LED port to double up as a CPU temperature display, perhaps only of real use if the PC's case has a side window.

A diagnostic LED port

The LED shown in the photo will display two alpha or numeral symbols according to what the BIOS is doing at that time.  Those symbols can then be looked-up in the motherboard's paper manual.  For example, if this particular Epox board was not initialising and the display showed "C1", a quick look in the manual tells me that the BIOS was trying to initialise the system RAM at the time of the halt.  I would then know to try some different memory modules, in the hope of fixing the problem.