What is a kippah?
A kippah is the Hebrew word for skullcap, also referred to in Yiddish as a yarmulke, or less frequently as a koppel. Jewish law requires men to cover their heads as a sign of respect and reverence for G‑d when praying or speaking G‑d's name while reciting a blessing, during study, and while in a synagogue or yeshiva. This practice has its roots in biblical times, where the priests in the Temple were instructed to cover their heads.
Traditionaly, Jewish men and boys wear the kippah at all times, a symbol of their awareness of, and submission to, a "higher" entity. There is no requirement either biblically nor explicitly stated in the Talmud that this practice be followed although the practice is noted in the Talmud. Yet, through the ages it became an accepted Jewish custom, which, according to the majority of halachic authorities, makes it mandatory. One should, therefore, not walk or even sit, bareheaded. Small children should also be taught to cover their heads.
Even those who do not wear a head covering at all times, will cover it as a sign of respect when attending religious services such as at a cemetery, a shiva house, or a wedding.
Many Jewish men and boys wear a kippah even while wearing a hat. The rational is that when the hat is removed either for comfort or politeness, the head remains covered.
While most chassidic Jews wear the traditional black kippah, many Jews will wear kippot (plural form of kippah) of various colors or designs. During the High Holidays, many wear white kippot. Some communities have developed kippah designs that are highly intricate works of art, such as the yemenite kippahs.
