Difference between Sadaqah & Zakat
Sadaqah is an entirely voluntary type of charity, that can be given at any time of year, and any amount can be given. However, Zakat is an obligatory charity due from every Muslim whose total possessions (cash, gold, silver, shares) meet or exceed the nisab threshold on a yearly basis.
Zakat has a number of stipulations regarding the type of assistance it can provide. However, Sadaqah can be used for any project or programme which is of benefit to people, whether its for the construction of a Masjid or used to dig a water well.
The Messenger of Allah ﷺ emphasised the benefits of giving Sadaqah in numerous Ahadith.