Manual Shifter Accessories

If you operate a stick-shift vehicle, you probably have a number of inquiries.

How does the strange "H" pattern this shift knob is travelling through relate to the transmission's gears? When I move the shifter, what is shifting within the transmission?

What exactly is grinding when I goof up and hear that awful grinding sound? What would happen if I unintentionally shifted into reverse while travelling at a high rate of speed along the freeway? Would the transmission blow up in its entirety?