The Taj Mahal is a mausoleum built as the resting place for emperor Shah Jahn's favorite wife.
The Hajj is an Islamic pilgrimage to Mecca that all Islamic followers must do at least once in their life if they are able to. The black stone in the middle is called the Kaaba. The Kaaba is considered the "house of God".
The star and crescent became a symbol of Islam in the 20th century after it started appearing on many flags such as the Ottoman empire.