According to Wikipedia, Ramadan is

...the ninth month of the Islamic calendar. It is the Islamic month of fasting, in which participating Muslims refrain from eating, drinking, sexual conduct, smoking, and indulging in anything that is in excess or ill-natured; from dawn until sunset. Fasting is meant to teach the Muslim patience, modesty and spirituality.

The Islamic calendar is lunar, whereas the Gregorian calendar is solar, so the beginning of Ramadan varies in date on the international calendar of record, while not varying on the Islamic calendar. This year, Ramadan begins on August 21st in the Eastern Hemisphere and on the 22nd in the Western Hemisphere. However, since practicing Muslims celebrate Holidays beginning in the evening of the previous day, Ramadan effectively begins at sundown tonight. For more information about Ramadan, take a look at these titles:

• Better than a thousand months : an American Muslim family celebration
• Ramadan
• Ramadan
• Ramadan : motivating believers to action : an interfaith perspective

Likewise, read and watch President Barack Obama's best wishes to the Muslim community at the White House website.
We also have materials about Islam and Islam -- Customs and Practices.