If you have always been curious about what exactly happens inside that "box" once your press the power button then today is your lucky day. There is a lot of information about it on the web but i think this blog has a very good explanation of it.

From the Article (FTA):

"Things start rolling when you press the power button on the computer (no! do tell!). Once the motherboard is powered up it initializes its own firmware - the chipset and other tidbits - and tries to get the CPU running."

Links:

How Computers Boot Up: Gustavo Duarte

What is booting up?

What is a BIOS (Basic Input/Output System?