What is "compound interest"?

Compound interest is interest calculated not only on your original deposit but also on all the interest that deposit has already earned, so your balance grows on an accelerating curve rather than a straight line. Left untouched over many years, even a modest starting amount can snowball substantially, because each year's gains become part of the base earning interest the following year, which is why starting to save early tends to matter more than the exact amount saved.

A one-time signup bonus is a single fixed payment unrelated to how your money grows over time, and a hidden bank fee is a cost taken from your account, the opposite of interest earned in your favor.

The dramatic long-term effect of compounding is often summed up in the idea that doubling time can be roughly estimated by dividing 72 by the interest rate, a quick mental shortcut investors use to picture how fast money can grow.