A crooner is a singer — usually a man, in the old popular style — who sings in a soft, gentle voice, especially about love. He leans into the microphone as if he were singing to you alone. The word belongs especially to the 1930s–1950s, the radio and early-record years, when that smooth style was a whole job. People still use it for anyone who sings that way, sometimes fondly, sometimes a little teasing, as if the singer were from another time. This is the person, not the action: the action is to croon.