There are several kinds of artificial lights - fluorescent, tungsten, LED, halogen, xenon, explosives, electric arc etc. And there are also several different kinds of natural lights - sunlight, moonlight (sunlight reflecting from the Moon), light from other stars, fire, lightning, volcanoes, aurora borealis, glowworms etc. Obviously, both classes contain very different light sources and any differences between such wide classes can only be found when overgeneralizing the classes to a couple of most common examples of both (e.g. xenon speedlight vs sunlight).

Most natural light sources are remarkably further away than reach of artificial light sources, therefore the drop in intensity (falloff) of artificial light is faster, since the light source is so much closer. Therefore, the area you could lit with a single artificial light is much smaller. Try lighting up a landscape, or the sky with a monolight :)

The most common forms of natural light - sunlight and moonlight - are always on, while artificial light sources most commonly used in photography are synchronized to switch on during exposure. Thus natural light provides for easier modeling of the lighting, and your camera's max sync speed is irrelevant, and there won't be any blinking to flash.

Dispersion of sunlight over the sky implies that the shadows cast by sun are not pitch black, but filled with a bluish tint.

Since artificial lights can be moved easily, you can easily create lighting schemes that would be impossible with natural light only (you might have some luck with directing fire or glowworms, not so much with others).

Finally, a few words on "quality" in business context (superiority), as opposed to philosophical context (property or attribute).

Here, artificial light thrives on

• availability (you can bring it any time, day or night);
• repeatability (you can get same lighting by using same setup again; the sun and moon are moving, weather might change);
• reliability (weather has much less effect on artificial light because there's much less of it between the light source and scene; with artificial light, uncharged batteries are your fault, not the lighting's).

Note that for artistic results, the unpredictability of natural light might be preferable.

Natural light easily beats artificial on

• expected lifetime;
• initial cost;
• running costs.

There's no difference in signal-to-noise ratio if the illumination level of subject is the same. Sunlight (especially non-diffused) will provide stronger illumination than most artificial lights, and therefore better signal-to-noise ratio; other natural lights are weaker than a flash close to subject.