The Ka’aba, (also known as Kaaba, Kabah, or Caaba) which literally means “the cube”, is a stone structure said to be built by the prophet Abraham for the sole purpose of worshiping God. The Ka’aba is located in the center of Islamic world- Mecca. This structure is the reason Muslims face Mecca during prayer, though they are not worshiping the granite structure. “Muslims do not worship the Ka'aba and its environs. Rather, it serves as a focal and unifying point among the Muslim people.”

TThe black stone is used as an idol. Many people over hundreds of years have touched, and prayed around the stone, which is most likely a meteorite (though no tests have been done). Today, it is not in the best condition, and it held together by a silver band. One thing I found interesting was that many people believe that it was originally white, but turned black by absorbing people's sins.
http://islam.about.com/od/mecca/p/kaaba.htm
http://www.submission.org/hajj/kaaba.html
http://atheism.about.com/od/muslimholysitestexts/p/KaabaMecca.htm