The Base curve is used to alter how the raw light data is interpreted. You can think of this almost as the input curves in other apps, they control how the raw data in the file gets pulled into the image editing space. This is often where fancy curves are used to simulate different types of film and such... that's very much what it's doing, adjusting the response of the film to the light; only digitally.

The Corrections curves are used more to control how the image is rendered out to the destination. This is very much more like varying the type of paper you're printing on. (especially in the B+W paper world.) Since this is just the raw conversion step on the way into gimp, that analogy breaks down a little... maybe it's more like how your film is developed into a negative.

In theory, both curves are capable of the same things. In reality, because of where they arrive in the pipeline, if you're doing ANY other changes with other tools in there, you probably want to do your adjustments with the Base curve, so that those other adjustments are applied after your curves.

disclaimer, I've not used ufraw in a long while, and the above is from memory.

THAT SAID... personally, I would only apply curves in the input process to do very basic adjustments, or to perform camera correction. For example, I have a Canon 20D, it's known to over blow red and under count green... so the camera correction for it very subtly corrects those two flaws. There are actually a number of curves for it from Canon... one for "natural", one for "technically accurate" etc... they've done testing in the lab and built profiles to do very specific things. I typically apply those at import just to normalize the image so that I know the image I'm working on is not tainted by what camera it was taken with. Once I have the image in my editing app (LightRoom for me, Gimp for you) THEN I use artistic curves to adjust the image to the way I want artistically. Why? typically the raw converteres aren't built for artistic play and undo/redo experimentation... they are built to convert raw to something editable, with a minimal adjustment to correct for the deficiencies of the camera.