The word sawm is derived from Syriac sawmo.

Literally, Sawm means 'to abstain'. In the terminology of Islamic law, Sawm means 'to abstain from eating, drinking and sexual intercourse.

Definition
Muslims are prohibited from eating, drinking, smoking, and engaging in sexual intercourse between a husband and his wife from dawn (fajr) to sunset (maghrib).

Some benefits of fasting
A Muslim keeps away from from some things that God has allowed for them, namely eating, drinking, and intercourse between husband and wife. However, if someone is sick, nursing, or traveling they are exempt from fasting. Also, during the day of fasting, the Muslim is obligated to keep themselves away from that which is not permissable. Otherwise, if they only keep themselves from that which is permitted (eating, drinking, and sexual intercourse between the husband and the wife) then they are only starving themselves. So a Muslim, during the day of fast must keep close from some of that which is Halaal (permissible) and stay away from that which is haram (forbidden) in order to gain piety.

Beginning the Fast
In accordance with traditions handed down from the Prophet Muhammad, Muslims eat a pre-dawn meal called the suhoor. All eating and drinking must be finished before sunrise. Unlike the zuhr and maghrib prayers, which have clear astronomical definitions (noon and sunset), there are several definitions used in practice for the timing of "true dawn" (al-fajr as-sadq), as mentioned in the hadith. These range from when the center of the sun is 12 to 21 degrees below the horizon [1] which equates to about 40 to 60 minutes before civil dawn. There are no restrictions on the morning meal other than the restrictions on Muslims diet. After completing the suhoor, Muslims recite the fajr prayer.

Prohibitions while fasting
Eating, drinking, smoking, and sexual intercourse are not allowed between dawn (fajr), and sunset (maghrib).

Certain Muslims are excused from fasting. According to the Qur'an, not fasting is permissible for those for whom fasting is potentially dangerous to their health, and to the elderly who are too weak to fast for extended periods of time. For example, diabetics are not expected to fast, nor are nursing and pregnant women.

Observing the fast is not permitted for menstruating women. However, when a woman's period has ceased, she must bathe and continue fasting. Any fasts broken/missed due to menstruation must be made up whenever she can before the next month of Ramadan (Muslim month of fasting). Women, at times when not menstruating, must fast as God indicates that all religious duties are ordained for both men and women.

Part of the Greater Jihad
It can also be done voluntarily, as part of the Greater Jihad. The Sawm can teach the believers patience and self-control. Most importantly the fast is also seen as a great sign of obedience by the believer to God. Faithful observance of the Sawm is believed to atone for personal faults and misdeeds and to help earn a place in paradise. It is also believed to be beneficial for personal conduct, to help control passions and temper, to provide time for meditation and to strengthen one's faith. Fasting also serves the purpose of cleansing the inner soul and freeing it of harm.