Adhan / Azan is defined as to inform & to make alert. But according to the Shariat it is a special method, with specific words, to make the people know & to invite them for obligatory prayers.
Importance of Adhan: A very favourite feature in prayers & it is liked by Allah. There is open announcement of oneness of Allah & His Holy Messenger. The glory & victory of Islam is evidenced in it.
Holy Prophet has said that a person delivering Azan will be admitted in Jannah immediately next to the Prophet & Martyres. A person delivering Azan will be confirmed a high place in the hereafter.