Sawm is fasting (refraining from eating, drinking & sex). Fasting during the Holy Islamic month of Ramadhan is one of the 5 pillars of Islam. Muslims fast every day in that month from dawn till dusk - effectively ~15 hours - and is not as difficult as people think. After dusk they can eat, drink and have relations.
Fasting is also done outside of this month and is considered a very good form of voluntary worship in devotion to God. A voluntary fast can last as many days as the person wills, but it must not fall on an eid day.
True Form: صوم
Explanation: Fasting isn't just refraining from food and drink. The purpose of fasting is more spiritual than physical and it is a means of devotion to God as well as self-discipline. Fasting purifies and trains the body and soul together. The Quran speaks about fasting:
[2.183] O you who believe! fasting is prescribed for you, as it was prescribed for those before you, so that you may guard (against evil).
[2.184] For a certain number of days; but whoever among you is sick or on a journey, then (he shall fast) a (like) number of other days; and those who are not able to do it may effect a redemption by feeding a poor man; so whoever does good spontaneously it is better for him; and that you fast is better for you if you know.
[2.185] The month of Ramazan is that in which the Quran was revealed, a guidance to men and clear proofs of the guidance and the distinction; therefore whoever of you is present in the month, he shall fast therein, and whoever is sick or upon a journey, then (he shall fast) a (like) number of other days; Allah desires ease for you, and He does not desire for you difficulty, and (He desires) that you should complete the number and that you should exalt the greatness of Allah for His having guided you and that you may give thanks.
[2.186] And when My servants ask you concerning Me, then surely I am very near; I answer the prayer of the suppliant when he calls on Me, so they should answer My call and believe in Me that they may walk in the right way.
[2.187] It is made lawful to you to go unto your wives on the night of the fast; they are an apparel for you and you are an apparel for them; Allah knew that you acted unfaithfully to yourselves, so He has turned to you (mercifully) and removed from you (this burden); so now be in contact with them and seek what Allah has ordained for you, and eat and drink until the whiteness of the day becomes distinct from the blackness of the night at dawn, then complete the fast till night, and have not contact with them while you keep to the mosques; these are the limits of Allah, so do not go near them. Thus does Allah make clear His communications for men that they may guard (against evil).
Islam is a very considerate religion and fasting is easy.
Fasting is very good for the human body as well. It gives the stomach and other organs a rest and alters the blood sugar levels so that excess energy is depleted, and then later renewed. It makes the body lean and the mind open. Fasting is a healthy change to the eating patterns of humans and it is beneficial in many ways.