Wikipedia
A savings stamp is a stamp issued by a government or other body to enable small amounts of money to be saved over time to accumulate a larger capital sum. The funds accumulated may then be used to make a larger purchase such as taking out a savings bond or to pay a large upcoming bill. Often issued in conjunction with post office run savings banks, savings stamps have also been issued by private companies. Supermarkets have issued the stamps to enable the spreading of large bills, package holiday companies have used them to enable customers to save for an annual holiday, and utilities companies have used the stamps to enable customers to spread the cost of their bills.
Savings stamps are not to be confused with trading stamps which provide a discount on goods purchased as part of a customer loyalty program.
In philately, savings stamps are regarded as a form of cinderella stamp.