Ultrasound is sound above 20,000 hertz, which is the upper limit of what humans can hear. Ultrasound's uses include: ultrasound scans(seeing through flesh with sound.), medicine to be transfered to the brain, sonic screwdriver, tractor beams. More on these on the uses pages.
Infrasound is sound below 20 hertz, which is the lower limit of of what humans can hear. Infrasound can be used to monitor the vibrations of earthquakes by measuring the time it takes for the sound to bounce of the ground and back to the speaker.