What is Ramadan?

The month of Ramadan is that in which was revealed the Quran; a guidance for mankind, and clear proofs of the guidance, and the criterion (of right and wrong). And whosoever of you is present, let him fast the month, and whosoever of you is sick or on a journey, a number of other days. Allah desires for you ease; He desires not hardship for you; and that you should complete the period, and that you should magnify Allah for having guided you, and that perhaps you may be thankful.[Quran 2:185]