I am not a pilot, but I've just had a skin cancer chopped out, so I have a little to contribute.
UVa and UVb are different parts of the ultraviolet spectrum--UVa closer to visible blue, UVb further away (shorter wavelength). The shorter the wavelength, broadly, the stronger the effects.
One major effect of UV is skin cancer: mostly basal skin carcinoma, which won't kill you, but can result in nasty ulcers. Australians and NZers get high exposure to UV (in part because of the ozone hole), and as a result have high rates of skin cancer. UV may also exacerbate melanoma, which will kill you, probably, but UV seems to have only an indirect effect on melanoma (it can mess up DNA, apparently, which bears thinking about).
High UV exposure may also cause cataracts, and some other eye problems.
Various things absorb UV, whilst transmitting visible light: ozone in the upper atmosphere, air, glass, some plastics. Glass and plastics can be treated to increase UV absorption.
High altitude, therefore, increases exposure to UV (I once lived on the NSW Great Dividing Range, at about 3,600' ASL: sunburn was a real problem). Shielding by glass or plastic will reduce exposure, to a varying extent, subject to thickness and precise composition of the transparencies.
Photochromic sunglasses go dark when exposed to UV light: they do not work satisfactorily inside a closed car, so I would imagine that the flight deck of an airliner, a fortiori, would NOT have a high UV level. On the other hand, a low wing light aircraft could be Nature's own tanning bed, depending on the material of the canopy. Apparently, polycarbonate is good at UV shielding. On the other hand, again, it is possible to get sunburned even through glass, if exposed long enough. Therefore it would seem sensible to take precautions, as seen in all the best photographs of Australasian glider pilots: broad-brimmed hat, good quality sunglasses, preferably wrap-around, and plenty of SPF 30+.
If in doubt, it can't be too hard to get a reliable UV meter on loan or hire.
Keep safe--half my contemporaries seem to be having BCCs cut out.