Compound interest means you earn interest not just on your original principal, but also on the interest that's already been added in previous periods โ this is what makes long-term investments grow faster than simple interest would suggest. The more frequently interest compounds (monthly vs quarterly vs yearly), the faster your money grows, because interest gets added to the principal more often, creating a slightly larger base for the next calculation each time.
Simple interest is calculated only on the original principal throughout the entire period, so it grows linearly. Compound interest is recalculated on the growing balance (principal + previously earned interest) each period, so it grows faster over time, especially over longer durations.
More frequent compounding (monthly beats quarterly beats yearly) gives a marginally higher return at the same nominal interest rate, since interest is added to the principal base more often โ though the difference is usually modest for typical bank products.
The compounding effect becomes dramatically more powerful over longer time horizons โ the difference between simple and compound growth is small in year 1 but can be substantial after 15-20+ years, which is why long-term investing benefits so much from starting early.