What are dissertation hours in the scope of degree requirements for PhD? At my school for a PhD I am required to complete a minimum of 72 credit hours of study(around 4 years of study). Of this ~42 are to be course work and ~30 is to be dissertation. 

So I know what a dissertation is but not in the context of "30 dissertation hours." For students and professors what has been your experience and what is actually done each semester while you are taking the dissertation hours. Do you go to class or is it a one on one basis with a selected professor or maybe just your adviser. Is there no class/group time, and I just do research into the area I have proposed for my dissertation?

Will be asking my adviser and graduate school dean about this but was also interested in the dynamic that other schools have in this area.
In my PhD program, I was required to register for various research and dissertation related courses to fill the required 9 credit hours to be a full-time student. Those course ranged from 3 to 5 credit hours depending on what I needed. Those courses never met, and I did not receive a letter grade for them. They seemed to have two purposes: 1. existing to fill my schedule after my actual coursework was done (so that I was listed as being enrolled as a full-time student and so that they had something to charge me for), and 2. giving my advisor credit with the department for supervising a research or dissertation student. The mostly seem to be a bureaucratic slight of hand. 

Edited to add: I was also paid 20 hours per week to work as a Graduate Research Assistant, but I basically worked about 40-50 hours per week on going to class (until I was done with my required hours) and working on my research. Towards the end, when I was writing my dissertation, it was more. The hours I was officially paid and the credit hours I took, never really aligned with much, except that a 3-credit hour actual course (like "Functional Analysis") met for 3 hours every week.