Compound interest
Compound interest is interest earned on both the original amount and the interest already added to it, so savings (and debts) grow faster over time than with simple interest.
A sum P at a yearly rate r compounded n times a year grows to P × (1 + r/n)^(n × t) after t years. The more often interest compounds, the slightly faster it grows; the rate and the time matter far more.
It works against you on debt: unpaid card interest joins the balance and is charged interest itself.