Not a dumb question! In my experience, each hospital will define the exact title and patient population of each unit. I have seen lots of overlap between the term CCU and CVICU. CCU may mean coronary care unit or mean critical care unit. The latter deals with all sorts of critically ill patients, not just patients with critical problems of their coronary arteries. Also, the CCU and CVICU may be two different units in one hospital but you are likely to deal overflow and/or be expected to float from one unit to another. For example, my facility has a neuro ICU and a cardiac ICU but, in reality, both units take on cardiac and neuro patients depending on bed availability. All staff are expected to float between the two too.
The line is blurred overall. If you are applying for jobs, your best bet is to ask the hospital what sort of patients they place in these units and what expectations they have for floating. Expect to be flexible and versatile.