First, it is the name of the ninth month of the lunar Islamic calendar.
Second, it is the designated month of fasting (from dawn until dusk). It can be compared to the Christian observance of Lent, as it is a time for inner reflection, renewed devotion to God and self-control.
Because the Islamic calendar is lunar, it does not track time the same that the solar calendar (such as the one in contemporary use) does. Each month begins with the sighting of the new (crescent) moon and lasts 19 or 20 days.
The Fiqh Council of North America uses astronomical calculation to determine the beginning of lunar months, with Mecca as the referent.
On the basis of this method Ramadan for the year 1433 AH begins Friday, July 20, 2012. Eid al-Fitr (the feast day marking the end of Ramadan) will be Sunday, August 19, 2012.