Adhan is the Islamic ritual call to prayer. It was explained to Cat Stevens (now Yusuf Islam) as "music for God." Stevens said, "I thought, music for God? I’d never heard that before. I’d heard of music for money, music for fame, music for personal power, but music for God!" Adhan is called out by the muezzin in the mosque, sometimes from a minaret, five times a day summoning Muslims for mandatory (fard) prayers (salah). There is a second call known as iqama (set up) that summons Muslims to line up for the beginning of the prayers. The main purpose behind the loud pronouncement of adhan five times a day in every mosque is to make available to everyone an easily intelligible summary of Islamic belief. It is intended to bring to the mind of every believer and non-believer the substance of Islamic beliefs, or its spiritual ideology. Loudspeakers are sometimes installed on minarets for the purpose.
Sunnis state that the adhan was not written or said by the Islamic prophet, Muhammad, but by one of his Sahabah (his companions). Shi'a sources state that it is Muhammad who, according to God's command, ordered the adhan as a means of calling Muslims to prayer. Shi'a Islam teaches that no one else contributed, or had any authority to contribute, towards the composition of the adhan.
The adhan (also spelled Athan, Azan, Ezan) is the Islamic call to prayer. The Muslim call to Friday public worship and to the five daily hours of prayer. It is proclaimed by the muezzin, a servant of the mosque chosen for good character, as he stands at the door or side of a small mosque or in the minaret of a large one. The adhan was originally a simple "Come to prayer," but, according to tradition, Muhammad consulted his followers with a view to investing the call with greater dignity. The matter was settled when 'Abd Allah ibn Zayd dreamed that the faithful should be summoned by a crier. The standard Sunnite adhan can be translated as: "Allah is most great. I testify that there is no god but Allah. I testify that Muhammad is the prophet of Allah. Come to prayer. Come to salvation. Allah is most great. There is no god but Allah." The first phrase is proclaimed four times, the final phrase once, and the others twice, the worshipers making a set response to each phrase.
It’s simply a way to alert people when they are busy; that the time for the prayer (“Salah” in Arabic; one of the 5 pillars of Islam) has come. Especially in the Muslim world; because in Islam, the five daily prayers are supposed to be carried out in congregation as much as one can. And so in the old days, before the development of technologies (alarm clocks, mobile phones, etc) that was a fast way to let people know. Recently, in the past fifty years or so, they started using loudspeakers on top of minarets to call the adhan. It’s a beautiful and enchanting sound and it’s just calling people to come and worship.