Things that invalidate the Fast

- Intercourse
- Masturbation
- Eating and drinking
- Anything that is regarded as coming under the same heading as eating and drinking
- Letting blood by means of cupping and the like
- Vomiting deliberately
- Menstruation and Nifaas 

-- Intercourse 
This is the most serious and the most sinful of the things that invalidate the fast. 
Narrated Abu Huraira:
While we were sitting with the Prophet (ﷺ) a man came and said, "O Allah's Messenger (ﷺ)! I have been ruined." Allah's Messenger (ﷺ) asked what was the matter with him. He replied "I had sexual intercourse with my wife while I was fasting." Allah's Messenger (ﷺ) asked him, "Can you afford to manumit a slave?" He replied in the negative. Allah's Messenger (ﷺ) asked him, "Can you fast for two successive months?" He replied in the negative. The Prophet (ﷺ) asked him, "Can you afford to feed sixty poor persons?" He replied in the negative. The Prophet (ﷺ) kept silent and while we were in that state, a big basket full of dates was brought to the Prophet (ﷺ). He asked, "Where is the questioner?" He replied, "I (am here)." The Prophet (ﷺ) said (to him), "Take this (basket of dates) and give it in charity." The man said, "Should I give it to a person poorer than I? By Allah; there is no family between its (i.e. Medina's) two mountains who are poorer than I." The Prophet (ﷺ) smiled till his premolar teeth became visible and then said, 'Feed your family with it."
Sahih al-Bukhari 1936

-- Masturbation
The evidence that masturbation is one of the things that invalidate the fast is the words of Allah in which He says of the fasting person:

Narrated Abu Huraira:
Allah's Messenger (ﷺ) said, (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 al-Bukhari 1894

Causing ejaculation comes under the heading of the desire which the fasting person gives up. 
Whoever masturbates during the day in Ramadan has to repent to Allah and refrain from eating and drinking for the rest of the day, and he has to make up that fast later on. 
If he starts to masturbate then stops without ejaculating, he has to repent, but his fast is still valid, and he does not have to make it up later because he did not ejaculate. The fasting person should keep away from everything that provokes desire and shun bad thoughts. 
With regard to the emission of madhiy (prostatic fluid), the most correct view is that it does not invalidate the fast. 

-- Eating and drinking
This refers to food or drink reaching the stomach via the mouth. 
If anything reaches the stomach via the nose, this is like eating or drinking. 

Asim bin Laqit bin Sabrah narrated:
From his father who said: "I said. 'O Messenger of Allah! Inform me about Wudu.' So he said: "Perform Wudu well, and go between the fingers, and perform Istinshaq (excessive amounts of water into the nose) extensively except when fasting."
Jami` at-Tirmidhi 788

If water reaching the stomach via the nose did not invalidate the fast, the Prophet (ﷺ) would not have told those who are fasting not to snuff up water deeply into the nose. 

-- Anything that is regarded as coming under the same heading as eating and drinking

This includes two things: 
- Transfusion of blood to one who is fasting – such as if he bleeds heavily and is given a blood transfusion. This invalidates the fast because blood is formed from food and drink.
- Receiving via a needle (as in the case of a drip) nourishing substances which take the place of food and drink, because this is the same as food and drink.
With regard to injections which do not replace food and drink, rather they are administered for the purpose of medical treatment – such as penicillin or insulin – or are given to energize the body, or for the purpose of vaccinations, these do not affect the fast, whether they are intravenous or intramuscular (injected into a vein or a muscle).
- Letting blood by means of cupping and the like

Narrated Shaddad b. Aws:
The Messenger of Allah (ﷺ) came to a man at al-Baqi' while he was cupping on the 18th of Ramadan; he (the Prophet) was holding my hand. Thereupon he said: A man who cups and a man who gets himself cupped break their fast.
Sunan Abu Dawud 2369

Donating blood comes under the same heading as cupping, because it affects the body in the same way.
Based on this, it is not permissible for a person who is fasting to donate blood unless it is essential, in which case it is permissible. In that case, the donor has broken his fast and must make up that day later on. 
If a person suffers a nosebleed, his fast is valid, because that happened involuntarily.
With regard to bleeding that results from extraction of a tooth, surgery or a blood test etc., that does not invalidate the fast because it is not cupping or something that is similar to cupping, unless it has an effect on the body similar to that of cupping. 

-- Vomiting deliberately

Narrated Abu Hurairah:
The Prophet (ﷺ) said: if one has a sudden attack of vomiting while one is fasting, no atonement is required of him, but if he vomits intentionally he must make atonement.
Sunan Abu Dawud 2380

Whoever vomits deliberately by sticking his finger in his throat, pressing his stomach, deliberately smelling something nasty or persisting in looking at something that makes him vomit, has to make up his fast later on. 
If his gorge rises, he should not suppress it, because that will harm him. 

-- Menstruation and Nifaas 

Narrated Abu Sa`id Al-Khudri:
The Prophet (ﷺ) said: “Is it not the case that when she gets her menses, she cannot pray or fast?” 
Sahih al-Bukhari 304 

When a woman sees the blood of her period or nifaas (post-partum bleeding), her fast becomes invalid even if that is one moment before sunset. 
If a woman feels that her period has started but no blood comes out until after sunset, her fast is still valid. 
If the bleeding of a woman who is menstruating or in nifaas ceases at night and she has the intention of fasting, then dawn comes before she does ghusl, the view of all the scholars is that her fast is valid.
It is preferable for a woman to keep to her natural cycle and to accept that which Allah has decreed for her, and not to take any medicine to prevent her period. She should accept what Allah has decreed for her of not fasting during her period, and make up those days later on. This is what the Mothers of the Believers and the women of the Salaf used to do. In addition, it has been medically proven that these means of preventing menstruation are harmful and many women have suffered menstrual irregularities as a result. If a woman takes pills and her period stops as a result, that is fine, she can fast and her fast is acceptable. 
These are things that invalidate the fast. All of them – apart from menses and nifaas – only invalidate the fast if three conditions are met: that the person was aware of the ruling and not ignorant of it; that he did it knowingly and not out of forgetfulness; and that he did it by choice and was not forced to do it.