Develop a program for prospective customers to show them how their savings could grow over time. The idea behind compound interest is that the interest you earn each year is added to your principal (starting amount), so that the balance doesn't just grow, it grows at an increasing rate. This is one of the most useful concepts in finance. It is the basis of everything from personal saving plans to long term growth in the stock market and accounts for the effects of inflation. It is thought to have originated in the 17th century and can be thought of as ‘interest on interest’.