Pronounced
PAY-us, these are the famous long
sidecurls of the Orthodox
Jews, worn in obedience to
Leviticus 19:27 in the
Torah / Bible:
You shall not round the corners of your heads, nor mar the edges of your beards.
"Corners of the head" is interpreted as the area above the ears, while "not rounding" them means neither
shaving the hair there nor cutting it particularly short. How short?
Long enough that you can grab a hair and bend it towards its own root - so not necessarily particularly long, as depending on your manual dexterity that can comes out to be just down to the middle of the ear, a far cry from the glorious "
do your ears hang low" spirals we know and love, but in striving to avoid only just making the minimum efforts to pass God's rules many wear them longer, flaunting the restriction as a badge of membership.
These more ostentatious fellows wind the curls carefully and let them hang conspicuously in front of the ears, while others tuck them behind the ears or up under their yarmulke. Payos are frequently paired with very short hair on the rest of the head (with the exception of the beard!) not for any scriptural reason but rather for practical reasons (easier to get those smoov black hats on with less hair in the way - no joke!) and again for group uniform purposes.
Most sects of Hasidim wear visible payos, with the exception of the adult Lubavitchers (the boys wear them, short, until they can grow beards, at which point they blend in)), and many non-Hasidic Orthodox groups wear them, including many Sephardic and Yemenite Jews.
Boys begin wearing payos at age three (before which, their hair is not cut at all). On their third birthday they take part in a special ceremony where their hair is cut short except for the three-years'-growth sidecurls coming down from the temples. He also receives his first set of tzitzit at this time and is no longer a considered a baby, but rather a child, member of a different category with more responsibilities.