What is the difference between sadaqah and zakat?

The Arabic term ‘sadaqah’ [voluntary charity] is one of a few words to which Islam ascribes a very broad meaning. It is often used interchangeably with zakat. Indeed, it is the term used to refer to zakat in the Quranic verse enumerating the categories of deserving zakat recipients. However, sadaqah is more general than zakat, because the latter denotes only a particular amount of property which a Muslim must pay as a duty. If he refrains from paying it, he incurs a grave sin. Indeed, Abu Bakr, the Prophet’s first successor as head of the Islamic state and his closest Companion, went to war against the Arab Bedouin tribes which refused to pay zakat after the Prophet’s death. He considered their refusal to pay zakat a rebellion against Islam and the Islamic State and said: "By God! I will fight those who differentiate between the prayer and the zakat."

Sadaqah refers to all charitable donations, whether obligatory or not. It is, however, more frequently used to refer to voluntary charitable donations. The meaning which immediately springs to mind when the term sadaqah is used, is the financial help offered to a poor person without any obligation on the part of the giver or any condition imposed on the recipient. In an authentic hadith, Abu Huraira quotes the Prophet (peace and blessings be upon him) as saying: "Every person’s every joint must perform a charity every day the sun comes up: to act justly between two people is a charity; to help a man with his mount, lifting him onto it or hoisting up his belongings onto it is a charity; a good word is a charity; every step you take to prayers is a charity; and removing a harmful thing from the road is a charity." In another version of the hadith, the Prophet (peace and blessings be upon him) was quoted to have mentioned another example of charitable action; he said: "To smile to your brother is a charity.”

Anyone who is familiar with Islamic philosophy can easily appreciate the great value attached to sadaqah. This is further reinforced by the fact that Islam does not confine sadaqah to financial help. Kindly actions and good turns done by one person to another are considered in the same light. The term is used in ordinary speech by all people, whether educated or not, to refer to any good and kindly deed, including the removal of harmful objects from the road.
And God the Almighty knows best.