Ditto.
But if you want a nerdie explanation, look on wiki.
According to it, metal originated from Blues rock and Psychedelic rock.

Are you for real or what?System of a down is metal?That is the funniest thing i've heard for a while.Apparently agonist and agony scene are metalcore,according to metal-archives.Your choice of bands to describe the epic journey that is metal disturbs me greatly.Sabbath and Megadeth...my granny could name those.

I personally think 2 things seperate metal from rock:complexity and heaviness.

Proper metal is complex,nuanced music.It has layers,and depth.It is played by skilled musicians,alot of whom are masters,and are peers of master classical musicians. Anybody can play rock,and even make a living from it.There are plenty of rock bands that make it big because of marketability,not talent. Like i said,anybody can play rock,it doesn't take alot of talent,just look at the charts.That's not to say there aren't rockers who are good enough to play metal music,there are plenty i'm sure.But you won't see meg white or the drummer from good charlotte playing any Tomas Haake material any time soon.

Heaviness is probably subjective but i think it is fairly easy to identify.Where you draw the line,who knows.It is blurred now by sub-genres such as post-rock/metal and post-hardcore.I think that is why people use myriad sub-genres to describe music.What is Isis,Tool,Fates Warning,Buried Inside?I call them all metal,because they have all done heavy,complex material,and to call them rock belittles them in my eyes.But plenty of people swear that Tool,Isis and Fates Warning are not metal bands.But to list in the same genre as Nickelback?No thanks.

I don't think it has to be complex and it definitely doesn't have to be nuanced to be metal. In fact a lot of metal has it's appeal in it's lack of nuance.

I'm guessing your a fan of Deathspell Omega?

Anyway, what you said has some truth to it, but I don't think metal is differentiable solely because of talent and complexity. Tons of doom metal bands aren't really complex or talented in the slightest, yet they're obviously still metal. Its really quite hard to make a clear cut line between rock and metal, but I would just say heaviness separates the two. Nothing more, nothing less.

Everything i said was in comparison to rock music,not stand alone

It all depends on what you yourself think what metal is. And what it boils down to is what started metal. Its the bands like Led Zeppelin, Deep Purple, and of course Black Sabbath. While nowadays they aren't really considered metal. But they were. And as you keep going forward in time you can see what has evolved from those bands. Those bands all developed from blues and classical. And you cans still see all of those elements in todays metal. To me personally it has to be bleusy to be metal. I think for it to be metal is has to have elements of everything previous pioneers have done to pave the way for metal. Metal is a direct descendant of blues in my opinion. And from that developed great bands such as the previously mentioned. And from that came great bands like Judas Priest, Iron Maiden etc. Then thrash metal came thanks to the likes of Metallica. It all have evolved and does sound alot differant from back in the day. But remaining the same roots has made metal what it today.

Pop, the word, is short for...?
Rock can be pop.

And to be fair, aside from the origins, metal is simply the darker side of rock. Hence the headbangers couldn't ever like something like The White Stripes, because TWS have yet to barb wire a lamb to a crucifix.

Of course System of a Down is considered a Metal band. Most of their albums are labeled alternative-metal. Their won best metal album awards, that's the section you'll find them in on Itunes.

To suggest otherwise is to be blind. Perception is reality.

Rock:


Metal:

Crash pretty much summed it up.