Short answer: it depends.
First, a bunch of disclaimers: I'm not a physician, and my work is much more on the molecular basis of cancer (rather than treatment), but I'll throw my 2 cents in.
As one of the earlier posters mentioned, "cancer" is really a bunch of different diseases that are grouped together under a single umbrella. What they have in common is that the delicate balancing act that characterizes our entire lives is disrupted: some cell type starts multiplying where or when it shouldn't.
Our bodies are constantly replacing certain cells that have naturally short lives (blood cells, for example); if that replacement process gets out of whack, and more are produced than need to be, you end up with something like a leukemia. In other cases, where the tissue is naturally long-lived and slow-dividing (such as the neurons in your brain), almost any extra proliferation can cause problems. (Such as a tumor.)
So why then is there not a single explanation or treatment for cancer? A drastic oversimplification that might be helpful is to picture the process like the rigging of an old-time sailing ship: hundreds of ropes, pulleys, sails, and people all working together. There are lots of things that can go wrong: a rope breaks, a pulley slips, someone trips and falls. The end result might be the same, in that the ship fails to go where it's supposed to, but the exact causes are different.
Now imagine that every single person on Earth is a slightly different ship, with similar but not identical ropes, crews, hulls, and sail materials, sailing different parts of the ocean, at different times of the year, in all different weather conditions. Some are well-maintained, some not.
So while we have a reasonable definition of cancer (too many cells where they shouldn't be) the causes (and fixes) can be quite different from type to type, or even person to person. Some are easier to treat because the root cause is known, and there's only one thing "broken"; others are more difficult because something complicated (or several complicated things) aren't working as they should.
So I have great hope that as we uncover patterns in how various cancers start and continue, there will be better and better treatments for certain types. I personally don't think there's a single general mechanism underlying all of these diseases, so there won't be a one-size-fits all "cure" for cancer anytime soon, but I can easily see a thousand different cures, eventually.