Muhammad was the prophet who founded and spread the Islamic religion.
The hajj is the Muslim pilgrimage to Mecca and is one of the Five Pillars of the Islamic religion. Believers who are able are required to make the hajj once in their lives.
A mosque is an Islamic house of worship. The Great Mosque in Mecca, called al-Haram, was originally built between A.D. 634-644 but has undergone many modifications. The Ka'aba is in the center of this mosque.
The Ka'aba is a cube-shaped religious monument in the middle of the Great Mosque. Pilgrims on the hajj walk around the Ka'aba seven times while repeating prayers.