Expansion Header/Interrupts

Hi All,

I'm new to the BB and I've only had my board for a few weeks now and
I'm trying to understand the Expansion Header/GPIO/Interrupts. I can't
seem to find any tutorials or any extensive information on this
subject?

I would like to trigger my board to turn on/wake from sleep when
triggered by an external sensor (through an interrupt, if that works
best). Can anyone point me in the right direction or suggest the best
method for triggering?

Thanks!