Originally called Yathrib, it was a city located about 200km north of Mecca. Unlike the Meccans who were hostile to Muhammad, the Madinans invited Muhammad there and accepted Islam when he came. The migration was known as the Hijra. The Muslim community was quickly organized around Madinah. From here, the character of the Qur'anic revelations began to change; from patiently inviting the Jews and Christians and pagans to Islam, to the sanction of using force against them. Raids on Meccan caravans were organized out of Madinah. At first this was diffult since they are relatives of the Meccans, but the Qur'anic commands were to be obeyed. Also, many different verses regulating the lives of Muslims began to appear. Madinah also became one of the centers of Islamic jurisprudence and culture.
Yahya related to me from Malik from Nuaym ibn Abdullah al-Mujmir that Abu Hurayra said, "The Messenger of Allah, may Allah bless him and grant him peace, said, ‘There are angels at the entries of Madina, and neither plague nor the Dajjal will enter it.’" (Malik's Muwwatta 45.45.4.16)It would be interesting to see if indeed Madinah had been kept from all plagues.
at-Taubah 9:101,120; al-Ahzab 33:60; al-Munafiqun 63:8