With an instalment loan (annuity loan), you repay the same amount every month. The instalment has two parts: interest on the outstanding amount and repayment of the loan.
At the start, the interest part is higher. With each instalment, the outstanding amount falls, and so does the interest part, while the repayment part rises. The instalment itself stays the same, which makes planning easy. With the last instalment, the loan is fully repaid.
You can find explanations of other terms in the credit glossary.