Interest looks like the simplest idea in finance until you compare two products. One quotes a nominal annual rate, another quotes an effective yield, a third quotes a monthly rate, and a fourth quotes an APR that folds in fees. None of those numbers are lies, but they are not measuring the same thing, and comparing them directly will lead you to the wrong product more often than not.
Simple interest is the honest starting point. It applies only to the original principal, so it grows in a straight line and never accelerates. That makes it the right model for a surprising number of real products, including many fixed term loans and some short deposits, and the wrong model for anything that reinvests. The simple interest calculator is worth running alongside a compounding projection on the same inputs, because seeing the two diverge is the fastest way to internalise what compounding is actually adding.
APR and APY exist to make comparison possible, and they solve different halves of the problem. APY tells you what a rate is worth once compounding is accounted for, which is what you want when you are the one earning. APR is designed to include costs, which is what you want when you are the one paying. Reading a borrowing product with the lending intuition, or the reverse, is a common and expensive mistake.
