What is alternating current?
First of all, the electric current , from the physical standpoint, is a flow of electrons along a conducting material, like copper wire.
When the direction of this flow is constant, it's a direct current . If the direction is changing (standard is 50 times per second in Europe and 60 times per second in the US), it's the alternate current .
The intensity of the direct current (physically, the number of electrons passing through the conductor in a unit of time) is constant, the intensity of the alternate current is changing from some maximum in one direction down to zero, then to some maximum in another direction, again down to zero etc. The graph of this process is a sinusoidal curve.
Technologically, the alternate current is easier to produce than the direct one and easier to use. Both production and usage are related to converting mechanical energy of rotation into electricity (production) and back (usage). Electric generators and motors are simpler if the alternate current is used.