Sound is what happens when a vibrating object, such as a ringing bell, sends a pressure wave through the atmosphere. When something vibrates in the atmosphere, it moves the air particles around it. These first air particles move the air particles around them, and the pulse spreads the vibration through the air in a wave that ripples outward -- just like what happens in water when a pebble is dropped.
Inside the human ear is a very thin membrane called the eardrum. When a sound wave reaches the ear, it vibrates the eardrum back and forth. The inner ear translates these fluctuations into an electrical signal and sends it to the brain. Our brain interprets this motion as sound.
Just like light waves, sound waves have their own spectrum of frequencies. A higher wave frequency means that the air pressure is vibrating faster and the ear hears this as a higher pitch. With lower frequencies, there are fewer vibrations in a given period of time, and the pitch is lower.
Human beings can hear sounds from 20 Hz (low frequency) to 20,000 Hz (high frequency). Dogs can hear sounds above 20,000 Hz, and mice can hear sounds as high as 98,000 Hz. Elephants, on the other hand, can hear more low-pitched tones than humans.
White noise is what results when all the different frequencies of tones the human ear can hear are combined together. It is similar to light, which is white when all the colors (frequencies) are combined.