In alternating current (a.c.) the movement of charge changes direction. The charge moves backwards and forwards, first one way and then the other, and this repeats many times each second. It does not travel steadily round the circuit in one direction as it does in d.c.
An alternating current is caused by an alternating voltage, which reverses its polarity again and again. One complete backwards and forwards movement is a cycle, and the number of cycles each second is the frequency, measured in hertz (Hz).
Unlike a battery, which supplies d.c., the mains supply in homes provides a.c. Mains a.c. is made by generators at power stations, and a lamp or heater works just as well on a.c. as on d.c., because energy is transferred whichever way the charge moves.