We know that Stun can kill - it's just a matter of exposure (STVI:TUC). Or a matter of victim's physical condition ("Conscience of the King" has Lenore grab a phaser and kill his frail father with it, even though the phaser was grabbed from a random guard who'd have his on stun, and she does not adjust anything). Yet Kill is always spoken of as a separate, somehow fundamentally different function. OTOH, Kill is a setting that also encompasses burn effects, hole effects and outright vaporization, while lethal Stun creates some bruising. So, separate phenomena, or just a matter of separate power levels...? One might argue that Kill involves maiming that typically amounts to burning holes - but sometimes the burning holes are completely inside the body, and only on occasion (an occasion of needless overkill by an untrained or reckless user) does the damage extend all the way to the outside. That is, the phaser effect goes "through" some things without visibly harming them and does the biggest damage deep inside the victim. If Kill is just very heavy Stun, then it's sort of confirmed that the effect can go "through" outer layers without visible effect, because Stun almost always does just that. Personally, I prefer to think that Stun is a completely different phenomenon from Kill, and that Kill is simply the same as Disintegrate - but a skilful killer can take a life with such minimal disintegration that we don't notice any marks. Humans are vulnerable like that: a bit of wrinkling on a shirt doesn't kill the shirt, but if the same sort of wrinkling is taking place at the heart and in the nerves surrounding it, the victim is a goner. It's not a precise science, though, and sometimes the disintegration of internal organs does create visible external damage. Timo Saloniemi
My explaination for the different phaser effects is that the stun settings and low kill setting have an effect on the target similar to an electric shock. Fired on a human being, the result is an overload of the central nervous system. This effect can range from a mild stunning effect to a literal frying of the nervous system, killing the person. The burn effect increases with the settings and when decreasing the distance to the target, explaining the burning on the crewman's forehead in ST6. On the highest settings, the burning (or heating) effect is more and more replaced by the disintegrating effect.
That's a real thing--but not quite as good as we see on screen yet http://en.wikipedia.org/wiki/Electrolaser When the beam gets close to metal, it jumps, looking like the DC Comics Excelsior's tracing phasers http://up-ship.com/blog/?p=15308