Fasting is a kind of worship for achieving self-discipline and self-cleansing, in which Muslims abstain from Halal (Lawful) things also such as all types of eatables, temptations, wrong-doings and sexual indulgence with their spouces, from dawn to dusk.
Muslims are enjoined to observe month long fasting in the month of Ramadan (9th month of Islamic calendar).

Arabic word for fasting is Sawm.
Sawm literally means to stop oneself from something.

While observing Sawm (Fasting), Muslims stop themselves from eating, drinking, smoking and having sexual intercourse with their spouses. The requirement of Sawm (Fasting) is not only to go hungry and thirsty during the day. The basic requirement of Sawm (Fasting) is total abstention and shielding from all sorts of evils and vices. Sawm (Fasting) requires a total cleansing of the body and mind.

While observing Sawm (Fasting), Muslims abstain from food, water, sexual desires, loose talk, lies, backbiting, unfair dealings and any other bad acts or desires.

Sawm (Fasting) helps in staying away from everything that is disliked by Allah (God).

Abu Hurayrah R narrated: The Prophet S said, "Whoever does not give up forged speech and evil actions, Allah is not in need of his leaving his food and drink (i.e. Allah will not accept his fasting.)"
Sahih Bukhari, Volume 3, Book 31, Number 127

Sawm (Fasting) is one of the traditionally mentioned five pillars of Islam, and an important and integral form of Islamic ‘Ibadah (Worship). Allah (God) had ordained fasting for the followers of other religions like the Jews and the Christians also, before Islam.

"O You who believe! Fasting is enjoined upon you in the same way as it was enjoined upon others before you so that you may become pious."
Surah (Chapter) al-Baqarah (The Cow) 2:183

Although fasting was prescribed for the Jews and the Christians also, but at present, fasting among them has been left as a symbolic gesture only.

Muslims observe Sawm (Fasting) from dawn to dusk, for one complete month, every year. The month of fasting is known as Ramadan and it is the ninth month of the Islamic calendar. Sawm (Fasting) is made obligatory by Allah (God) during the month of Ramadan.

Besides increases Taqwa (Fear of Allah, Righteousness), Sawm (Fasting) also shields Muslims from evil acts. It gives them a total control over their desires.

Abu Hurayrah R narrated: Allah's Apostle S said, "Fasting is a shield (or a screen or a shelter). So, the person observing fasting should avoid sexual relation with his wife and should not behave foolishly and impudently, and if somebody fights with him or abuses him, he should tell him twice, 'I am fasting." The Prophet added, "By Him in Whose Hands my soul is, the smell coming out from the mouth of a fasting person is better in the sight of Allah than the smell of musk. (Allah says about the fasting person), 'He has left his food, drink and desires for My sake. The fast is for Me. So I will reward (the fasting person) for it and the reward of good deeds is multiplied ten times."
Sahih Bukhari, Volume 3, Book 31, Number 118

Every Muslim must observe fasting for one complete month every year.
Month of Ramadan has a special significance because the revelation of the Holy Qur-an began in this month, when the Messenger of Allah, Muhammad S received the very first Wahi (Message from Allah).

Qur-an specifies the month in which fasts are to be observed.

"The month of Ramadan in which was revealed the Qur-an, a guidance for mankind and clear proofs for the guidance and the criterion. So whoever of you sights the moon, he must observe fast in that month, and whoever is ill or on a journey, the same number (of days which one did not observe fast, must be made up) from other days. Allah intends for you ease, and He does not want to make things difficult for you. He wants that you must complete the same number of days, and that you must magnify Allah for having guided you so that you may be grateful to Him."
Surah (Chapter) Al-Baqarah (The Cow) Qur-an 2:185

Missing a fast without any serious sickness or any other valid reason is a grave sin.

Allah (God) has created human beings for the purpose of His worship by performing acts that please Him. Humans are Khalifah (Representative) of Allah (God) in this world. They must try their best to reflect good. Self discipline and self-control is essential for the Islamic way of life. Fasting not only disciplines a person but also inculcates self-control in that person.

There are many people in this world that end up wasting more food in a party than many could afford in an entire year. Such people do not have any understanding of the problems of the poor or the needy. They cannot comprehend the feelings of the needy and poor, because they have never undergone the same type of pain due to hunger and thirst.
The more fortunate have understand and be more considerate to the needy, if they, themselves were to go through some of the circumstances that these people have to pass through every day. He made fasting obligatory so that everyone should feel hunger and thirst so that whenever one comes across a hungry or thirsty person, one should have at least some sympathy for them. This was the philosophy behind fasting to which Allah refers in His Book in the following words:
"Perhaps you may become pious."

"O You who believe! Fasting is enjoined upon you in the same way as it was enjoined upon others before you so that you may become pious."
Surah (Chapter) al-Baqarah (The Cow) 2:183

The Arabic word Tattaqun means to inculcate the habit of fear in oneself of the consequences of the Hereafter, and become a perfect human being. It means that everyone should always have the consequences of their dealings before their eyes. One will reap whatever one sows. We should not ignore the hunger and thirst of the needy and the poor despite being resourceful, we may also be totally ignored by Allah (God) Who had bestowed His bounties upon us.