Chromatic abberations are a deviance of the light going through the lens based on it's spectrum wavelength.
Confused yet?
Light travels at different wavelengths (Ultraviolet to Infrared). Each wave has its own speed and properties. Sometimes, lenses make it so that certain wavelengths deviate from the line their supposed to follow: this results in "fringing", which is a coloured halo around edges, particularly where dark and light areas meet.
Many cameras correct for this in-camera (all the newest Nikons, for instance), but for some it has to be done in post, which is where RAW comes in real handy. Try getting rid of CA on a jpg and you'll understand why.
What it boils down to is that it's inherent in a lens: generally it affects wide-angle more than telephoto. I know the NIkon 10.5 fisheye is VERY bad with CA in the edges and corners and it frustrated me no end. But it's otherwise a GREAT lens. It's hard to avoid it: you can buy your lenses based on their CA characteristics, but that's boring as all hell and won't leave you with much of any interest. Just learn to live it, correct it, and enjoy shooting.
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