Click or Indexed shifting: Any new bike with gears and the old shifters used on internal geared hubs were “click” shifters or indexed. Meaning there are definite, marked stopping points on the shifter where it “clicks” into gear.
Friction: Friction shifters have no definite stopping points or positions. You just keep moving the shift lever until you feel and hear the bike shift to a different gear. Friction shifters were generally used on old “10 speed” style bikes with a dérailleur.