They are specific ultra violet rays.
UVB are the rays that help your body (and your chameleon's) to produce Vitamin D3.
This vitamin is essential in helping your body utilize Calcium (bone growth and density)
Sorry for keeping it so simple ... but I do think that covers it.
uvb only penetrates the epidermis (of humans, that is,) and uva can penetrate into the dermis -the inner layer of your skin...
this page explains it perfectly! http://en.wikipedia.org/wiki/Ultraviolet
Natural sources of UV
The Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the atmosphere's ozone layer, 99% of the ultraviolet radiation that reaches the Earth's surface is UVA. (Some of the UVB and UVC radiation is responsible for the generation of the ozone layer.)
Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths while Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.[2][3][4]
The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque below this wavelength. This opacity is due to the strong absorption of light of these wavelengths by oxygen in the air. Pure nitrogen (less than about 10 ppm oxygen) is transparent to wavelengths in the range of about 150–200 nm. This has wide practical significance now that semiconductor manufacturing processes are using wavelengths shorter than 200 nm. By working in oxygen-free gas, the equipment does not have to be built to withstand the pressure differences required to work in a vacuum. Some other scientific instruments, such as circular dichroism spectrometers, are also commonly nitrogen purged and operate in this spectral region.
Extreme UV is characterized by a transition in the physics of interaction with matter: wavelengths longer than about 30 nm interact mainly with the chemical valence electrons of matter, while wavelengths shorter than that interact mainly with inner shell electrons and nuclei. The long end of the EUV/XUV spectrum is set by a prominent He+ spectral line at 30.4nm. XUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50% of XUV radiation at normal incidence. This technology has been used to make telescopes for solar imaging; it was pioneered by the NIXT and MSSTA sounding rockets in the 1990s; (current examples are SOHO/EIT and TRACE) and for nanolithography (printing of traces and devices on microchips).
A positive effect of UVB exposure is that it induces the production of vitamin D in the skin. It has been estimated that tens of thousands of premature deaths occur in the United States annually from a range of cancers due to vitamin D deficiency.[5] Another effect of vitamin D deficiency is osteomalacia (the adult equivalent of rickets), which can result in bone pain, difficulty in weight bearing and sometimes fractures. Other studies show most people get adequate Vitamin D through food and incidental exposure.[6]
Many countries have fortified certain foods with Vitamin D to prevent deficiency. Eating fortified foods or taking a dietary supplement pill is usually preferred to UVB exposure, due to the increased risk of skin cancer from UV radiation.[6]
Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as psoriasis and vitiligo. UVA radiation can be used in conjunction with psoralens (PUVA treatment). UVB radiation is rarely used in conjunction with psoralens. In cases of psoriasis and vitiligo, UV light with wavelength of 311 nm is most effective
It is also used for calcium and vitamin D3 absorbtion, above explains some of it. UVA is used in plants.... I might have mixed them up but I'm pretty sure that UVA is for plants and UVB is for Animals. Also, UVB has some parts in Plants
I must say this was a pretty precise description (I personally am of the "more information is always good" school )
To complete the above mentioned, and as I have read further literature to complete my answers for the Repti Glo concerning post, it can be added that UVB is far better a D3 and other liposoluble vitamin source than any commercial vitamin powder or solution. Indeed, vitamin absorption in the body is quite poor for humans in the form of pills or powders - that is, synthetic vitamins - and is even believed to be worse for reptiles. According to these authors, and it is the general observation in human medicine too, a vitamin on its own is mostly evacuated in renal filtration, whereas vitamin combinations as those found in natural food are far better at being absorbed by the organism.
Furthermore, I have read many times that UVA is also important in animal life, notably, for triggering specific behaviors - it has been known to enhance reproductive behavior as well as improve appetite. It's probably providing the animal a more naturalistic "feeling" about the environment than of any direct chemical use, but it is needed as well.
Last but not least, it is the UVA spectrum that triggers melanosynthesis in the human body - namely, tanning. UVAs induce melanine and other pigment production, which is our natural barrier against too high UV doses, and is also responsible for skin coloration. UVA could be implicated in the animal's coloration and the "shiny" colors then.
As a chemist, I can't stress how much synthetic vitamins are poor at absorption, and this is precisely why UV light is much needed, especially when breeding species that are accustomed to direct exposure to sunlight.
It is also an important factor in hypervitaminosis, as dosing precisely the vitamins is hard when they are partly flushed out of the system.
Therefore, UVB light is necessary for healthy metabolism, UVA should be healthy in a more indirect way, and vitamins are a complement that should be used with much attention to any sign of overdose - gut feeding the preys is an important step towards avoiding excessive use of vitamin powders.
Hope the addition was of some use
PS: for plants, UVAs are an important growth factor, but the main spectrum is in the visible rays, except for green which is obviously the absorption minimum. A neon tube emitting high in reds and blues, down to UVAs, is healthy for green plants as chlorophylle needs those wavelengths to produce glucose and oxygen in great amounts.
Second Edit:
UV overdoses can be carcinogenic or mutagen. Eye damage is common for overexposure to strong UVB lights, burns occur in most UV overexposures. Keep that in mind when considering another tube that those that have proven useful for chameleons (namely the 5.0 ones).
UVC radiation is filtered by ozone and is barely noticeable on the ground level. As it is highly mutagen, UVC sources are used for sterilization in extra-pure atmosphere laboratories.
This is to say that it is necessary to respect the "strength" of tube to be used and not exceed the tube use duration, as degrading tube coatings favor UVC transmission and lower UVB and UVA production.