A.C. stands for Alternating Current, a current that changes it's polarity as well as magnitude with time. When the graph of current is plotted against time you shall get a graph of sine function.
A.C. has a rms value, which is normally written on devices and used in calculation of power consumed, a peak value which is the maximum value of current in a complete cycle and an average value which is average of all the values of current in a time period.