Did you check out
www.sirweb.org, they can explain the field a lot better than I can, and they even have a video clip about interventional radiology.
I really don't know a whole lot about radiation oncology, as its a different specialty that radiology.
You have an incredible source of knowledge literally at your finger tips. Google it, man! If you still don't know what you want to know ask. As far as interventional radiology goes, its a subspecialty whose importance is difficult to understand outside of the context of the rest of medicine. Check out the website, learn a little more about medicine in general if you have specific questions feel free to ask.
-Hans