Compare alternating current with direct current: what do they have in common and how do they differ?

The main feature of direct current is that it is constant both in magnitude and in sign. Direct current “flows” to one side all the time. For example, along metal wires from the positive terminal of the voltage source to the negative terminal (positive and negative ions create it in electrolytes). The electrons themselves move from minus to plus.
The alternating current changes during the period both its value and the direction of the movement of electrons. In our household sockets, this is a current with sinusoidal (harmonic) oscillations at a frequency of 50 hertz (50 oscillations per second).