Sidelocks (Peyot) in Jewish tradition
Ye shall not round the corners of your heads. Leviticus 19:27
Peyot lit. sidelocks are the long extra hairs at either side of the head worn mostly by Charedi Ashkenazim and Yemenite Jews.
The custom is apparently ancient. It is interesting to note that in Yemen they were never called peyot but rather simanim literally signs, apparently it was some sort of hairstyle to make it easier to tell the Jews apart from their Arab neighbors.
Yemenite Rabbi Yosef Kapach claims that it was an ancient custom dating back to Temple days but then again he is not exactly an unbiased source on this.
Monroe Rosenthal in his book "Wars of the Jews" (a fascinating book by the way) mentions the Jewish (convert) king of Yemen, Yusuf Dhu Nuwas who wore his sidelocks long "as was the custom of the princes of the Davidic royal house". (looking for source)
The Otzar Yisrael encyclopedia by Yehuda David Eisenstein writes that to the Jews of the Middle East and North Africa this custom was unknown and foreign.
Rabbi Yosef Chaim of Baghdad (Ben Ish Chai)however, was a strong proponent of it writing in one of his works " I do not propose wearing long sidelocks as our Ashkenazi brethren in Poland and Galicia do but.."
Some Chassidic and non-Chassidic groups have the custom of hiding the sidelocks behind the ears or under the skullcap. The origins of this curious phenomenon are not known for certain. I think it had something to do with anti-Jewish persecutions in some parts of Europe. I am also reminded of the Zoharic prohibition against the peyot harosh (sidelocks) coming in contact with the peyot hazaken (beard) due to mystical reasons.