Q: What is the difference between Sadaka and Zakat ?

A: Zakaah in Arabic means growth, blessing and purification.
See Lisaan al-‘Arab, 14/358;, 2/399
Sadaqah comes from the word sidq (sincerity), i.e. it is a sign of sincerity of faith on the part of the person who gives it.
See Fath al-Qadeer, 2/399
With regard to the shar’i definition:
Zakat means worshipping Allah by giving that which He has enjoined of different kinds of Zakat to those who are entitled to them, according to the guidelines prescribed in sharee’ah.
Sadaqah means worshipping Allah by giving money without that being made obligatory in sharee’ah. The word sadaqah is sometimes used to refer to obligatory Zakat.
With regard to the difference between Zakat and sadaqah, it is as follows:
1 – Zakaah is enjoined in Islam on specific things, which are: gold, silver, crops, fruits, trade goods and an’aam livestock, i.e., camels, cattle and sheep.
With regard to sadaqah, it is not obligatory on any kind of wealth, rather it is what a person can give, without any specific limits or guidelines.
2 – Zakaah is subject to the conditions that one full Hijri have passed since acquiring the wealth, and that the wealth meet the minimum threshold (nisaab), and it is a specific portion of wealth.
Sadaqah is not subject to any conditions, and it may be given at any time, in any amount.
3 – Allah has enjoined that Zakat be given to certain types of people, and it is not permissible to give it to anyone else. They are the people mentioned in the verse (interpretation of the meaning):
With regard to sadaqah, it may be given to those mentioned in the verse on Zakat and to others. 
4 – Whoever dies and owes zakaah, his heirs must pay it from his wealth, and that takes precedence over the will (wasiyah) and inheritance.
As for sadaqah, there are no such obligations with regard to it.
5– According to the four schools of law, it is not permissible to give Zakat to one’s ascendants or descendents. Ascendants include one’s mother, father, grandfathers and grandmothers; descendents include one's children and their children.
Sadaqah may be given to one's ascendants and descendants.
6 – It is not permissible to give Zakat to one who is rich or who is strong and able to earn a living.
Sadaqah may be given to those who are rich and those who are strong and able to earn.
7 – In the case of Zakat, it is better for it to be taken from the rich of a land and given to their poor. Many scholars are of the view that it is not permissible to send it to another country unless that serves an interest.
But charity may be spent on those who are near and those who are far.
8– It is not permissible to give Zakat to kuffaar and mushrikeen.
Sadaqah may be given to kuffaar and mushrikeen.
Allaah says (interpretation of the meaning):
 “And they give food, in spite of their love for it (or for the love of Him), to the Miskeen (the poor), the orphan, and the captive” [al-Insaan 76:8]
Al-Qurtubi said: In dar al-Islam (the Muslim lands), a captive could only be a mushrik.
9 – It is not permissible for a Muslim to give Zakat to his wife. Ibn al-Mundhir narrated that there was scholarly consensus on this point.
But sadaqah may be given to one’s wife.