Alternating current is a flow of charge that changes its direction periodically. That's why the voltage level also reverses along with the current. Alternating current is directly delivered to households, offices, commercial spaces etc.
Different wave graphs can be used to specify alternating current. The most usual form of electric power circuit is Sine wave form.
The frequency of AC powers supply is varies from country to country and sometime also within the country. Most AC electric power is generated at the frequency of either 50 hertz or 60 hertz. Some countries also supply the mixture of 50 hertz and 60 hertz frequency of AC. Low frequency AC is preferred over high frequency because low frequency is used in electric motors, crushing and rolling applications, and railways motors etc while high frequency causes acceleration of electric charge which produces electromagnetic radiation.