What is Zakat-ul-Fitr?
Zakat-ul-Fitr (also called Sadaqat-ul-Fitr) is an obligatory charity for a Muslim who possesses minimum wealth. Literally, fitr means breaking the fast or creation. It is called the "head zakat" because it is a financial responsibility for each person. 

In a hadith, the Prophet (may Allah's peace and blessing be upon him) said: 
"The fasting of the month of fasting will be hanging between earth and heavens, and it will not be raised up to the Divine Presence without paying the Zakat al-Fitr."

Who has to pay it?
Any Muslim whose wealth is in excess of the nisab must pay zakat-ul-fitr. A Muslim, wealthy enough to be eligible to pay zakat-ul-fitr must also pay it for children and the insane who lack financial resources and are living in his custody or under his care. 
Additionally, grandchildren whose father has passed away must also be paid zakat-ul-fitr. 

Do children need to pay zakat-ul-fitr?
Zakat-ul-Fitr should be paid by the head of the household on behalf of babies and children. 

When should it be paid?
Zakat-ul-Fitr becomes obligatory on the first day of Ramadan, with the break of dawn. It is advised to pay Zakat-ul-fitr at least two days before Eid. The latest one can pay is before the Eid prayer. 

Ibn Abbas narrates:
'It is accepted as Zakat for the person who gives it before the Eid prayer, but it is a mere Sadaqah for the one who gives it after the prayer'. (Abu Dawud)

Delaying paying zakat-ul-fitr?
Zakat-ul-Fitr must be paid before the Eid prayer. If paid after that, it is considered general charity and a person blameworthy for that. 

Can it be paid in cash?
At the time of the Prophet (saw), zakat-ul-fitr would be given as one Sa'a (approximately a handful) of a staple food to those in need. Abu Saeed narrates:

'We used to give for Zakat al-Fitr…one Sa'a of food, or one Sa'a of dried yoghurt, or one Sa'a of barley, or one Sa'a of dates, or one Sa'a of raisins.' (Bukhari)

Today's monetary value is based on the price of a staple food such as flour or rice, which is around £5. Therefore, the amount due for each person is £5.

How should you intend to pay?
Zakat-ul-Fitr is an act of worship, and thus it necessitates correct intention. When you pay online, at the time of paying, just make the intention before you hit the play button, Insha'Allah. The heart intends that the zakat-ul-fitr is for the sake of Allah. 

Wisdoms of Zakat-ul-Fitr?
Zakat-ul-Fitr is prescribed on Eid-al-Fitr because Eid is Allah's great symbol (sha'air). It is a means of purification for those who fasted. Zakat-ul-Fitr compensates for any shortcomings of fasting. It makes up for any weaknesses in their fasting, just as the sunnah prayers compensate for defects in the fard prayers. 

Empathy with the less fortunate:
The rich and those who can afford to display and enjoy Eid, but the poor remain the same at Eid. Therefore, Allah has ordained that zakat-ul-fitr be paid as an expression of empathy and compassion for the blessings. This small amount of zakat-ul-fitr enables the poor to enjoy some benefit during this great occasion. We must remember that wealth is an Amanah, and it does not belong to us.