Interstellar gas

We detect hydrogen gas in the ISM in the form of neutral, ionised and molecular form. In astronomy, these are denoted as HI (neutral), HII (ionised), and H$_2$ (molecular), which is far from ideal. You would think HII to be double ionised - impossible of course - it means H$^+$. And when speaking, you cannot distinguish HII from H$_2$ - both are pronounced as H-two. So, CIV is triply ionised carbon. I'm sorry about this!

How much is there of each type, and more importantly, why do we sometimes find one form, sometimes the other?