Interest
Interest is the price of money: what a borrower pays, and what a saver earns.
In plain words
Interest is usually given as a rate per year. At 4% a year, €1,000 earns or costs €40 in a year.
Both sides
The bank pays you interest on savings and charges you interest on loans. It lives on the gap between the two.
Fixed or variable
A fixed rate stays the same for the whole loan. A variable rate moves up and down with the rates set by the central bank (in the euro area, the European Central Bank, or ECB).
Monthly rates hide the real cost
A card charging 1.5% a month costs about 19.6% a year, not 18%, because each month's interest is charged on the last month's interest too.