well, if you wanted a small definitional difference, here ya go!
Sonar (Sound Navigation And Ranging) is typically associated with underwater acoustics, and it is common to find low and mid frequency sonars that are audible to the human ear (eg you can hear the "ping" of a sonar against a subarine hull), but there are ultrasonic sonars also, like the ones used to find fish on a fishing boat.
Ultrasonics are the frequencies above the level of human hearing, >20Khz.