A sound has something called a frequency. It refers to how many times a second the sound wave makes the air vibrate. A sound wave with a frequency of 20 Hz makes the air vibrate twenty times per second. 

Humans can hear sounds with a frequency of 20-20000 Hz. That is known as the audible range. Ultrasonic waves are above 20,000 Hz, and infrasonic are below 20 Hz.
Sound waves with frequencies below 20 Hz are called Infrasonic.
Sound waves with frequencies above 20,000Hz are called Ultrasonic.
We cannot hear infrasonic of ultrasonic sound waves, but many animals can. Bats, dogs, dolphins, can hear ultrasonic sounds. Elephants and giraffes can hear infrasonic sounds.