Hey I've been looking into some tabs, and i came across this 'Natural Harmonic' and 'Artificial Harmonic'

What are these ? In this tab i'm looking at they make this A on G string (2nd fret) sound like a high A (like 17fret E string) or some... and this other time sounds like high C or some......

What's the deal behind this and how u play it? :P :P

Also, is this the same thing as u hear in metal riffs sometimes ? (other riffs too ofc :p) How do u get that sound ? Like in 'in and above men' by moonspell u can hear it quite often

Is this the same ? If not, how do u play that :P

Also, (even dumber question now:P), I saw 'Harm' on some tabs aswell, do they just mean play hard ? Ye i know, nubish questions xd

Thanks in advance
naturals are sweet spots over the frets by lightly touching the string and normally plucking and quickly removing your frettin finger to let it ring. most easily done on the 3rd, 5th, 7th, and 12th frets. artificial harmonics, a.k.a. pinch harmonics, are where you normally play a string, but as you pluck the string, you tap your thump lightly against the string. these are a little tougher as you have to get the technique down and find the right sweet spots over the pickups. + idk if it's just my guitar, but if you have a 2 humbucker guitar and a 3-way switch, it's easier to hear the pinches on either the neck or bridge, not both.
ok guys, natural harmonic seem to work quite well, pinch harmonics gonna take me some while i think

But thanks ! xd