standard European color codes for power cables
I gather that you hail from the world of low voltage electronics, breadboard spaghetti and all that 5 volt stuff.  They use the term "GND" or "ground" to refer to Vss, the current return or common, typically the negative in a 5/12V system.  (which actually makes it the current source, but nevermind that.)

Here, we work in AC, because it is transmittable over long distances.  That changes a whole bunch of stuff, also it can kill you. 

Equipment Safety Ground

Separate from the power conductors proper, the "positive" and "negative" as you call them (wrongly)... 

... we also wrap the entire kaboodle in a metallic safety shield.  This safety shield is not a conductor (meaning: power should never flow through it normally.)  If it does, something is dreadfully wrong, and the goal of the metal is to keep the misguided current flow away from humans.  And also to allow enough current to return to source that it trips circuit protection systems - overcurrent trip devices (circuit breakers) or ground fault detection systems (GFCI/RCD). 

What we call "ground" in mains wiring refers to that safety shield.  This is a very different thing than in DC electronics.

AC: blowing your mind twice

Positive and negative are not concepts in mains wiring -- at least not half the time, since it is AC.  AC itself is bizarre idea to understand - why reverse polarity 16.67, 25, 50, 60 or 400 times a second???? Why bother? The generator is barely simpler - slip rings instead of a commutator. The reason is transmissibility.  It goes through transformers without outlandishly expensive M-G sets all over the place. 

Of course, being AC, what happens if two feed wires are out of phase?  Hoo boy.  There's a whole science just to syncing two generators (once sync-locked, push-pull forces keep them in sync, except solar, whose digital synthesis can't be pushed).  

Mind-blow #2: What if they're out of phase intentionally?   Great for motors -- just like you can't start a bicycle if you're at top dead center, a motor can't start on single-phase without some electromic trickery.  Having 2+ phases means having 2+ cranks so one is always in the power stroke.   There's no point having two phases 180 degrees out of sync with each other - you could swap the two wires and have the same effect.   So a 2-phase system is 90 degrees apart, and takes 4 wires - two for phase A, and two for phase B.  If you drew it on graph paper, thinking of the phase as a vector, it would be a plus sign.  

A 3-phase system has each phase 120 degrees apart (or 60 degrees if you also consider mirror "phases".)  Drawn on paper, you could draw it like an asterisk and take 6 wires.  But you could also draw it like a "Y" (wye) or a triangle (delta) -- and now you are down to 4 or even 3 corners!  That's right, you can do  the 3-phase "delta" configuration with only 3 wires.  Now, next time you're out and about, look up at the power poles and what do you see, up there on top?  BINGO.  You see wires in threes.  (you might also see a lightning arrestor wire.)  

But you won't see 3-phase in residential unless you're in Europe, Asia, Africa, Australia or South America.  There, they deliver 3 phase to your pole, but in a "wye" (4 wire) configuration. Each phase is 230V from neutral.  They only deliver to your house one phase and neutral. So your house sees 230V single-phase.

Some large European houses get 2 or 3 phases delivered to the house, but this is simply to provide more power.  Individual circuits are still songle-phase.  Very heavy-draw appliances like stoves or on-demand water heaters can be wired to spread their load across 2 or 3 phases.  And of course if the homeowner has a serious home machine shop, he will make good use of 3-phase for motors.  

What does polarity mean if it's AC?

We don't want the output side of the transformer rattling and floating at crazy voltages.  For instance if there was leakage in the 9600V transformer which serves your neighborhood, it could inadvertantly pull your 230V service up to 9600 volts from earth - which would overwhelm normal wire insulation and kill everyone in the neighborhood.   So the transformer's 230V output is biased (by way of a grounding rod) to keep its voltages from drifting too far from earth potential.  (other than of course their intended voltage).  Often, the bias is done very simply by a grounding strap, so it is a 0 volt bias.  You could have a 3-volt bias if you really wanted to, but 0 is simpler. 

In single-phase, you have two wires, and you bias/bond one of them.  

Now suddenly the system has a polarity: one wire is biased near ground, and the other one operates at a significant voltage from ground.  Take an Edison style screw-in bulb socket: the easily-touched screw-threaded part of the socket should be which one? The one near earth hopefully! 

The one near earth is called the neutral.  In the US, the lawyers call it a "grounded conductor", since it is a conductor, and it is the one that is biased/bonded/pegged near ground.  

Neutral is not ground and should never be abused as ground or vice versa.  I'm talking to you, NEMA 10!  Nothing really enforces this except ambulances and RCD/GFCI devices.