عَنْ أَبِي عَبْدِ الرَّحْمَنِ عَبْدِ اللَّهِ بْنِ عُمَرَ بْنِ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: سَمِعْت رَسُولَ اللَّهِ صلى الله عليه و سلم يَقُولُ: " بُنِيَ الْإِسْلَامُ عَلَى خَمْسٍ: شَهَادَةِ أَنْ لَا إلَهَ إلَّا اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ، وَإِقَامِ الصَّلَاةِ، وَإِيتَاءِ الزَّكَاةِ، وَحَجِّ الْبَيْتِ، وَصَوْمِ رَمَضَانَ
On the authority of Abu Abdur-Rahmaan 'Abdullah, the son of 'Umar ibn al-Khattaab رضي الله عنهما who said: I heard the Messenger of Allah صلى الله عليه وسلم say: "Islam is built upon five [pillars ]: testifying that there is none worthy of worship except Allaah and that Muhammad is the Messenger of Allah, establishing the prayers, giving the zakat, making the pilgrimage to the House and fasting the month of Ramadhaan." 
Linguistically, siyaam means to abstain from something, such as abstaining from speaking. In the sharee'ah, it is a direct reference to abstaining from food, drink and sexual intercourse during the days of the month of Ramadhaan. This practice is one of the pillars of Islaam, as noted in this hadeeth.
Fasting is a source of self-restraint, piety and God-consciousness. It was prescribed by Allaah سبحانه و تعالى for the prophets عليهم الصلاة والسلام before Prophet Muhammad صلى الله عليه وسلم. In the verses obligating the fast of the month of Ramadhaan, Allaah سبحانه و تعالى has pointed out its goal or purpose:
"O believers! Fasting is prescribed for you as it was prescribed for those before you, that you may attain taqwa  [self-restraint, piety and God-consciousness]." (Sooratul-Baqarah, 2:183)
The Prophet صلى الله عليه وسلم said that fasting is a protection from the Hell-fire:
الصيام جنة من النار كجنة أحدكم من القتال
"Fasting is a shield from the Hell-fire like one of your shields shielding you while fighting." 
Furthermore, it will also come as an intercessor on the Day of Judgment. The Prophet صلى الله عليه وسلم has said,
الصِّيَامُ وَالْقُرْآنُ يَشْفَعَانِ لِلْعَبْدِ يَوْمَ الْقِيَامَةِ ، يَقُولُ الصِّيَامُ : أَيْ رَبِّ مَنَعْتُهُ الطَّعَامَ وَالشَّهَوَاتِ بِالنَّهَارِ فَشَفِّعْنِي فِيهِ . وَيَقُولُ الْقُرْآنُ : مَنَعْتُهُ النَّوْمَ بِاللَّيْلِ فَشَفِّعْنِي فِيهِ . قَالَ : فَيُشَفَّعَانِ
"The fast and the Qur'aan shall come as intercessors on the Day of Resurrection. The fast shall say, 'O Lord, I prevented him from his food and drink during the day, so let me intercede for him.' The Qur'aan will say, 'I kept him from sleep during the night, so let me intercede for him.' Then they will be allowed to intercede." 
It is an act that demonstrates one's sincerity to Allaah سبحانه و تعالى. Only Allaah سبحانه و تعالى is aware if a person truly fasted or not. No one can know if he secretly broke his fast. Therefore, Allaah سبحانه و تعالى has a special reward for those who fast. This is stated in the following hadeeth qudsi ,
يترك طعامه وشرابه وشهوته من أجلي الصيام لي وأنا أجزي به والحسنة بعشر أمثالها
Allaah سبحانه و تعالى has said, "He leaves his food, drink and desires because of Me. Fasting is for My sake and I shall reward it. And every good deed shall be rewarded tenfold." 
By Allah's grace and mercy سبحانه و تعالى, if a person fasts the month of Ramadhaan with faith in Allaah سبحانه و تعالى and hoping for its reward, Allaah سبحانه و تعالى will forgive all of his previous minor sins. The Prophet صلى الله عليه وسلم said,
مَنْ صَامَ رَمَضَانَ إِيمَانًا وَاحْتِسَابًا غُفِرَ لَهُ مَا تَقَدَّمَ مِنْ ذَنْبِهِ
"Whoever fasts the month of Ramadhaan with faith and hoping for its reward shall have all of his previous sins forgiven for him." 
Imaam lbn al-Qayyim رحمه الله noted some of the beneficial and important aspects of fasting when he wrote,
"The purpose of fasting is that the spirit of man was released from the clutches of desires and moderation prevailed in his carnal self, and, through it, he realized the goal of purification and everlasting felicity. It is aimed at curtailing the intensity of desire and lust by means of hunger and thirst, at inducing man to realize how many were there in the world like him who had to go even without a small quantity of food, at making it difficult for the Devil to deceive him, and at restraining his organs from turning towards things in which there was the loss of both worlds. Fasting, thus, is the bridle of the God-fearing, the shield of the crusaders and the discipline of the virtuous." 
There is also a hadeeth of the Prophet صلى الله عليه وسلم that warns of the punishment for one who breaks his fast improperly. In this hadeeth, the Prophet صلى الله عليه وسلم said,
بينا أنا نائم إذ أتاني رجلان فأخذا بضبعيّ ( الضبع هو العضد ) فأتيا بي جبلا وعِرا ، فقالا : اصعد . فقلت : إني لا أطيقه . فقالا : إنا سنسهله لك . فصعدت حتى إذا كنت في سواء الجبل إذا بأصوات شديدة ، قلت : ما هذه الأصوات ؟ قالوا : هذا عواء أهل النار . ثم انطلق بي فإذا أنا بقوم معلقين بعراقيبهم ، مشققة أشداقهم ، تسيل أشداقهم دما ، قلت : من هؤلاء ؟ قال : هؤلاء الذين يفطرون قبل تحلة صومهم
"While I was sleeping, two men came to me and took hold of my arms. They brought me to a steep mountain and said, 'Climb.' I said, 'I am not able to.' They said, 'We will make it easy for you. ' So I climbed until I came to the summit of the mountain where I heard terrible cries. I said, 'What are these cries?' They said, 'Those are the cries of the inhabitants of the Fire. ' Then they took me further until I came to a people who were strung up by their hamstrings, and their jawbones were torn and flowing with blood. I said, 'Who are these people?' He said, 'Those are the people who break their fast before the time it was permissible to do so."'   
 In their translation of this hadeeth, Ibrahim and Johnson-Davies make the following comment, "The word 'pillars' does not appear in the Arabic but has been supplied for clarity of meaning. Pillars (arkaan) is the generally accepted term in this context." Ibrahim and Johnson-Davies, p. 34, fn. 1. The narration from 'Abdur-Razzaaq explicitly mentions "supports" or "props". In the narration in Saheeh Muslim, five is in the feminine form, which would imply arkaan or pillars. Cf., Imaam lbn Hajr رحمه الله, Fat-hul-Baari, vol. 1, p. 72.
 This hadeeth has been narrated through numerous chains from 'Abdullah ibn 'Umar ibn al-Khattaab رضي الله عنهما. It was recorded by al-Bukhaari, Muslim, Ahmad, an-Nasaa'ee, at-Tirmidhi, al-Baihaqi (in three different works), ibn Khuzaima in his Saheeh, ibn Hibbaan in his Saheeh, al-Tabaraani in his al-Kabeer and numerous others. The hadeeth has also been narrated from the Companion Jareer ibn 'Abdillaah al-Bajali رضي الله عنه. Through Jareer رضي الله عنه it was recorded by at-Tabaraani with a saheeh chain. See: Shaykh Fauzi ibn Muhammad حفظه الله, Al-Adhwaa as-Samaawiyyah fee Takhreej Ahaadeeth al-Arba'een an-Nawawiyyah, pp. 57-59.
 For a definition of the word 'Taqwa' as well as an explanation of who the true 'Muttaqeen' are, see: Fear Allah Wherever You Are (Taqwa) - https://www.muftisays.com/forums/12-virtues/7703-fear-allah-wherever-you-are-taqwa.html
 Recorded by Ahmad, an-Nasaa'ee and others. According to Shaykh al-Albaani رحمه الله, it is saheeh. Saheeh al-Jaami, vol. 2, p. 720.
 Recorded by Ahmad. According to Shaykh al-Albaani رحمه الله, it is saheeh. Saheeh al-Jaami, vol. 2, p. 720.
 For more on Hadeeth Qudsi, see: https://www.muftisays.com/forums/12-virtues/9570-hadith-qudsi.html
 Recorded by al-Bukhaari.
 Recorded by al-Bukhaari and Muslim.
 Quoted in Shaykh Abul Hasan 'Ali Nadwi رحمه الله, The Four Pillars of Islam, p. 173.
 Recorded by ibn Hibbaan and ibn Khuzaima. According to Shaykh al-Albaani رحمه الله, it is saheeh. Saheeh at-Targheeb wat-Tarheeb, vol. 1, p. 420.
 Shaykh Jamaluddin Zarabozo حفظه الله, Commentary on the Forty Hadith of al-Nawawi, vol. 1, p. 279 and pp. 316-319.