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). This is a slightly confusing convention: you would think HII to be double ionised hydrogen - impossible of course - it means H$^+$. And when talking, you cannot distinguish HII from H$_2$ - both are pronounced as H-two. So, CIV is triply ionised carbon.

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