When you buy a stock, you're buying a small ownership stake in the company. When you buy a bond, you're lending the company (or government) money. That distinction drives almost everything else.
As a stockholder, you own a piece of the business. Your upside is unlimited if the company grows, but you're last in line if things go wrong. If the company goes bankrupt, bondholders and other creditors get paid first from whatever assets remain, and stockholders often get little or nothing. Stocks may pay dividends, but those aren't guaranteed and can be cut at any time.
As a bondholder, you're a creditor, not an owner. The company promises to pay you a fixed interest rate (the coupon) on a set schedule and return your principal at maturity. That payment is contractually owed to you, which is why bonds are generally less risky than stocks. But your upside is capped, you get your interest and principal back, nothing more, even if the company does spectacularly well.
This is also why bonds are more sensitive to interest rates. When rates rise, existing bonds with lower fixed rates become less attractive, so their market price falls. Stocks are more sensitive to the company's actual business performance and growth prospects.
In short: stocks are ownership with variable, uncapped returns and higher risk. Bonds are debt with fixed, capped returns and (generally) lower risk. Most long-term portfolios hold a mix of both to balance growth potential against stability.