A440
Since 1955 the international standard has been A = 440 Hz. It's the note the oboe gives for the orchestra to tune to, and the pitch of most tuning forks.
Why 442?
Many European orchestras and quite a few bands tune slightly higher, to 442 or 443 Hz, for a brighter sound. If your group tunes that way, pick it in the selector.
How to use it
Click the A so it plays continuously, play your note and adjust until the beating disappears (the slow wobble you hear when two notes are almost the same).
Frequently asked questions
Can I use it to tune a choir?
Yes, give the A and find each voice's starting note from it.