Direct Current (DC) is current whose direction is constant and unchanging.
Alternating Current (AC) is current whose direction changes from one way then back, repeatedly.
Both AC and DC currents are produced when an AC or DC power supply are connected to a circuit.
AC and DC voltages can be analysed with an oscilloscope, which shows graphically how voltage changes with time.
The height of a graph on an oscilloscope is a measure of its voltage.
The width of an AC graph on an oscilloscope is the time for the cycle to repeat. The shorter the time, the higher the frequency.