Is it just a brand logo? Is it an ancient symbol of the moon goddess that the Turks unwittingly imported into Islam? Or of the pre-Islamic moon goddess of southern Arabia? Or does it have a symbolic value - a secret - that has now become obscure and unfamiliar to modern Muslims? Modern Turks will themselves tell you that the crescent and star is an ancient Turkoman symbol that their nomad ancestors carried across Central Asia and established as the banner of Islam upon adopting the faith. Is it then merely an ethnic symbol after all? Perhaps its association with Islam is accidental and it has no profound meaning? We must remember that modernity is a forgetting. The hilal emerged as the emblem of Islam from the tumult of the collapse of the Caliphate and thus from the religion´s passage into modernity. Is it a symbol that survived from the closing phases of classical Islamic civilization but of which the significance has been forgotten?

There is certainly no consensus of opinion as to what the symbol actually means. This is why there are so many different variations used on flags and other Muslim insignia. The common elements are a crescent and a star, but there is no standard way to arrange them, except to say that the star is very often intruding into or is placed within the dark of the moon´s face. How does this represent Islam? What does it mean? The crescent alone is easy to explain: it represents the crescent moons by which the Muslim calendar is calculated, the hilal moon. Sighting and calculating the hilal is a Muslim past time today and in previous centuries was a serious scientific endeavour. It is understandable and appropriate as an emblem of Islam, and indeed the crescent is often seen portrayed alone, as in the Red Crescent of the emergency humanitarian organisation, the Muslim equivalent to the International Red Cross. But the crescent is usually accompanied by a star, and this creates difficulties. Which star? Which astronomical (or astrological) configuration is being depicted? And how does this appropriately represent Islam? It seems there are no clear answers to any of these questions.

http://www.theabodeofpeace.com/hilal.html