Sadaqah is an entirely voluntary charity that can be performed 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.
Essentially, the difference between Zakat and Sadaqah is that Zakat is obligatory and one of the five pillars of Islam while Sadaqah is simply a kind gesture made with the intention of helping others.
Zakat has a number of stipulations regarding the type of assistance it can provide. However, Sadaqa can be used for any project or program which is of benefit to people.