Frequency and pitch
Frequency is how many times a wave vibrates each second; pitch is how high or low we hear a sound.
Frequency is the number of vibrations a wave makes each second, measured in hertz (Hz). A sound at 440 Hz pushes the air back and forth 440 times a second. Pitch is how high or low that sound seems to us. The two are closely linked, since higher frequencies sound higher, but they are not the same thing: frequency is a physical measurement, while pitch is something the brain perceives.
The 440 Hz note is the A that orchestras usually tune to. Double the frequency and you get the same note an octave higher, at 880 Hz; halve it and you drop an octave to 220 Hz. Every octave is a 2:1 ratio of frequencies, which is why a piano keyboard repeats the same pattern of notes.
Human hearing is usually said to run from about 20 Hz up to about 20,000 Hz (20 kHz), though the upper limit falls with age. Below about 20 Hz, vibrations stop sounding like a note and are called infrasound. Above 20 kHz is ultrasound, used by bats and by medical scanners.
A common confusion is between pitch and loudness. Loudness depends on how big the pressure swings are, not how often they come, so a quiet note and a loud one can have exactly the same pitch.
See it in action
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