A kippah (literally: dome) 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, studying Torah, saying a blessing or entering a synagogue.
This practice has its roots in biblical times, when the priests in the Temple were instructed to cover their heads.
Traditionally, Jewish men and boys wear the kippah at all times, a symbol of their awareness of, and submission to, a "higher" entity.
Although it is not explicitly required by law, the practice is noted in the Talmud, and through the ages, this became an accepted Jewish custom to the point that according to the majority of halachic authorities, it is mandatory. One should, therefore, not walk or even sit, bareheaded. Small children should also be taught to cover their heads.
Aside from the commonblack kippah, many wear kippot (plural form of kippah) of various colors or designs. Some communities have developed kippah designs that are highly intricate works of art, such as those made by Jewish artisans from Yemen and Georgia, most of whom now live in Israel.
For more on the Kippah, click here and here.