Why do Jewish men wear yarmulkes (beanies) to cover their heads?

The yarmulke, meaning skullcap in Yiddish, is a beanie that covers a Jewish man's head. The Hebrew term for it is Kippah. Jews cover their heads during prayer, eating and studying as a sign of respect toward God, who is above you. Religious Jews will wear it all the time. Covering the head is more of a custom than a commandment.

Orthodox women cover their heads (some with a yarmulke) as a sign of modesty, since a woman's hair is considered tempting and seductive. Lately, more and more liberal women wear yarmulkes or head coverings, yet they do so as a sign of respect toward GodYarmulke.

When do they have to wear them?
Jews wear them when they pray, eat and study.