AC is a form of oscillating power, like a jig saw. The generator uses magnetic induction which, as you know has two poles + and - . One hertz, or one rotation creates one complete cycle but one cycle is comprised of both + and - forces.
Also one cycle is considered a sine wave, or sinusoidal wave.
A phase is considered a separate sine wave. If you live in the US, the secondary windings on the transformer that delivers power to your house uses one sine wave that is tapped in the center and is also 180 degrees of phase, which gives the center tap zero potential voltage which is considered the neutral.
Also, voltage and current are fundamentally different things. The voltage is a force carrier, it represents the magnetic induction, it wants to travels at the speed of light, as it is basically electromagnetism. Current is the transfer of mass, which the copper wire contains in a efficient makeup to allow the circuit to work.
If you want to read up the matter see
http://www.electronics-tutorials.ws/accircuits/sinusoidal-waveform.html