Tuning by ear with a tuning fork
Play the reference note and the same note on your instrument: if the two sounds are slightly apart, you hear beats, a wobble in the volume. The bigger the gap, the faster the beats. Turn the tuning peg until they disappear: the two notes are then in tune.
The sine (pure) sound is closest to a real tuning fork. The organ and bright sounds, richer in harmonics, are easier to hear over an instrument.
A = 440 Hz, 442 Hz or 432 Hz?
A at 440 Hz has been the international standard since 1939 (ISO 16). Many orchestras tune a little higher, to 442 or 443 Hz, for a brighter sound. 432 Hz has its fans, but there is no evidence that it is more “natural”. Set the reference to match the musicians you play with: it is shared with the site’s tuner.
Frequency generator
The slider covers the whole range of human hearing, from 20 Hz to 20,000 Hz, or you can type an exact frequency. Handy for testing a speaker or headphones, finding a room’s resonance or checking how high you can hear. Always start at a low volume, especially with headphones.