In an AC wave theoretically voltage and current in an AC rise and fall simultaneously. But in actual case there is a time interval between voltage and currents. This is call phase difference.
This phase difference is also occurs between two ac voltages also.
The phase difference is measured either in second or its submultiples such as milli or micro seconds or by Degrees.
The term leading and lagging is used to denote one wave is moving in front of other (leading) and if it is behind the other wave is called lagging.
Note1. Voltage and current phase difference is because of the inductive and capacitive reactance in an ac circuit.
Note 2. In a three phase ac circuit phase difference between each line is 120 Degree (angles)