RELIGION. The religion of Muslims is called Islam اسلام and the laws of God Shari’ah شريعة. There are three words used by Muslim writers for the word religion, namely, Din, Millah etc. and Muzhab. In the Kitaba ‘l-Ta’rifat the difference between these words is as follows:-
Din دينis used for religion as it stands in relation to God. e.g. Dinu’llah,. “the religion of God.”
Millah ملة, as it stands in relation to the Prophet or lawgiver, e.g.Millatu Ibrahim “the religion of Abraham,” or Millatu ‘r-Rasul “the Prophet’s religion.”
Mazhab مذهب, as it stands in relation to the decisions of the Mujtahidua e.g. Muzkabu Abi Hanifah.
The expression Din, however, is of general application, whilst Millah and Mazhab are restricted in their use. [ISLAM.]

Based on Hughes, Dictionary of Islam