What Is "Call to Prayer"?

I have been wondering what "call to prayer" is since I first came across this expression when listening to Studio Classroom on October 1. Entitled "Magical Marrakesh," that article mentioned that the haunting call to prayer from Koutoubia Mosque can be heard five times a day. While listening, I paid undivided attention, hoping that I would hear the teachers' explanation of this phrase. However, the teachers didn't clarify the term.

Still, the phrase has kept haunting me. Just now, I found two web pages where the phrase is not only explained but also "demonstrated" through videos. According to the two web pages, "call to prayer" can only be found in Islamic countires.  It is heard five times every day from a mosque to remind poeple to pray.  

http://tw.myblog.yahoo.com/kenlin-kenlin/article?mid=5020&prev=5045&next=5012&page=1&sc=1

http://blog.roodo.com/hocheleben/archives/5839279.html

More videos of  "call to prayer" are available at YouTube.   Just key in the three words on the search bar and you can listen to a few.   Another new experience.  How thankful I am to the Internet!  It makes life easier.