Compound interest is earning interest on interest. A full explanation can be followed on Wikipedia.

The calculator on the left allows to calculate the future value, given the other three variables. Alternatively, you can calculate the periodic rate (rate of return) by setting the other 3 variables and periodic rate to ZERO.

Amortization

Paying off a car or home loan is where the concept of amortization is applied.

Given a loan of 1'000'000, at 15% annual interest rate over 20 years, how much would the repayment be each month?

Although, if you're looking for a slightly more advanced amortization calculator, then take a peek at Morty (pre-beta). Morty's been incubated at Heroku but very much (bare-bones) functional.

Doubling Time

Doubling time (or halving time) is the length of time it'll take your money to double (or halve) in value.

For doubling, the assumed context is an investment; but halving is mostly applied in the context of inflation.