How interest is actually calculated, rather than what it grows into. Compounding frequency, simple versus compound, and why the rate a bank advertises is not the rate you receive.
Simple interest is charged on the original amount only. Compound interest is charged on the balance including interest already added, which is why it accelerates. Most real products compound, which is why the difference between them widens dramatically over long periods.
How often it compounds matters too. Daily compounding beats monthly, which beats annual, even at an identical quoted rate. That is exactly what APY exists to capture: it converts a nominal rate into the effective annual figure, so two accounts advertising different rates and different compounding can finally be compared honestly. In the UK the same idea is called AER.