What is a loan

A loan is an arrangement where one party gives money to another and gets it back later, with extra. The extra is interest. The amount given up front is principal. The deadline for getting it all back is maturity. The schedule for paying it back is repayment.

That's the whole structure.

Mortgages, corporate bonds, credit cards, and the trillions of dollars in private credit floating around institutional balance sheets all use these four pieces, scaled and recombined. Understanding loans well means understanding what each piece does and how they trade off against each other.

Principal: the amount on the table

Principal is the original sum borrowed. A $10,000 loan has $10,000 in principal the moment it opens. As the borrower repays, the outstanding principal shrinks. When it hits zero, the loan is closed.

Interest is calculated on the outstanding principal, not the original amount.

So a $10,000 loan that's half repaid is accruing interest on $5,000, not $10,000. That's why early repayments save money: every dollar of principal paid early is a dollar that's not generating interest for the rest of the loan's life. This single mechanic shapes how borrowers think about prepayment penalties, refinancing, and the difference between paying extra each month versus making one annual lump-sum payment.

Some loans pay off principal gradually. Most mortgages work this way.

Others pay all the principal at the end in a single bullet payment, which is how most corporate bonds and a lot of private credit work. Bullet loans concentrate repayment risk at maturity. Amortizing loans spread it out.

Interest: the price of time

Interest is what the borrower pays for the privilege of using the lender's money. It's expressed as a rate, usually annualized.

A 6% loan means the borrower owes 6% of the outstanding principal per year, charged in periods (monthly, quarterly, semiannually) depending on the loan.

Why does interest exist? Three reasons stacked together. The lender's money has alternative uses, so the rate covers the opportunity cost of not deploying it elsewhere. The lender takes credit risk that the borrower won't repay, so the rate compensates for that risk. And inflation erodes the value of money over time, so the rate needs to outpace it.

Different loans price these three components differently.

A 30-year US Treasury bond pays a low rate because the credit risk is near zero and inflation is the main thing being priced. A subprime auto loan pays a high rate because the credit risk is real and the lender knows a meaningful slice of borrowers will default. A junk corporate bond pays higher still.

Interest comes in two shapes: fixed and floating. Fixed locks the rate at origination. Floating ties it to a benchmark like SOFR.

Fixed gives the borrower predictability. Floating gives the lender protection. Most institutional loans are floating because the alternative leaves lenders exposed to inflation and rate cycles over multi-year horizons.

Maturity: the deadline

Maturity is the date the loan ends. By that date, all principal must be repaid and all interest paid. Loans get categorized by maturity: short-term (under a year), medium-term (1 to 5 years), long-term (5+ years).

Maturity sets the borrower's repayment pressure and the lender's exposure window.

A 30-year mortgage gives the borrower a long runway but locks the lender's capital up for decades. A 90-day trade-receivables loan turns over fast and frees the lender to redeploy quickly. The choice depends on what the loan is for and who's funding it.

Repayment: how the money flows back

Repayment is the schedule. It tells the borrower when to pay, how much each time, and what proportion of each payment goes to principal versus interest.

Three common shapes show up across most loans.

Amortizing loans (most mortgages, most car loans) spread principal repayment evenly across the loan's life. Each payment is the same dollar amount, but early payments are mostly interest and late payments are mostly principal.

Interest-only loans pay only interest until maturity, when the full principal comes due in one bullet. Common in commercial real estate and corporate debt.

Balloon loans pay small amounts of principal during the term and a large amount at maturity. A hybrid between the two.

The choice of structure shapes the borrower's cash flow and the lender's risk profile. Lenders care about repayment schedules because the schedule tells them when their money returns and how exposed they are if the borrower defaults somewhere in the middle of the term.

For deeper reading, see Secured vs unsecured lending: what collateral actually is and APR vs APY: why the difference matters.