UVA – this is part of the visible spectrum for reptiles, & because of it, they can see colors & patterns differently than we do. In the wild, it helps them identify individuals of their own species by their UVA-reflective markings as well as better spot certain plants and insects. When exposed to UVA, reptiles have shown increased social behavior & activity, more inclination to bask & feed, as well as breed. UVB – helps the production of vitamin D3 (vD3) in most reptiles. If I remember correctly, UVB rays stimulate a chemical (or hormone?) in the reptile's skin called ergosterol, which basically triggers the production of vD3. Deprived of this light, reptiles are at risk of developing a vD3-deficiency, which can be crippling & often fatal if not remedied.
Most regular "heat bulbs" or basking bulbs sold in pet stores will advertise that they provide UVA rays, but not UVB.
Only the tube-florescent bulbs and mercury vapor bulbs provide adequate amounts of UVB in an artificial captive environment. (of course, the compact "spiral-shaped" florescent bulbs do, too...but they are generally not suggested for tortoises as they have been know to cause blindness).