Duplex printing refers to printing on both sides of a sheet of paper or double-sided printing. Simplex printing refers to printing on only one side of the paper or single-sided printing.
Printers with duplex capability can automatically print double-sided pages by using a duplex feature. Printers with simplex-only capability can still print in duplex mode, but this requires the user to manually turn the print job over and start the printing again for the second side.
Duplex paper or media is designed to be printed on either side or both sides.