The two main things you can buy on a securities exchange represent opposite relationships with the issuer. A share makes you an owner. A bond makes you a lender. Almost every practical difference follows from that.
The bond: a lender's position
When you buy a bond you lend money to the issuer, who agrees to pay you interest at set intervals and return the principal on a stated date. The return is defined in advance. If the issuer meets its obligations, you know exactly what you will receive and when.
Your risk is that the issuer fails to pay, known as credit risk, and that interest rates rise, making your fixed coupon less attractive and pushing down the price if you need to sell before maturity. As a lender you rank ahead of shareholders if the issuer becomes insolvent.
The share: an owner's position
When you buy a share you take on the fortunes of the business. There is no defined return and no repayment date. If the company prospers, your claim on its earnings grows and there is no ceiling on that. If it fails, you rank last and can lose everything.
The trade-off
Shares have historically offered higher long-run returns than bonds, and that is compensation for accepting greater uncertainty and a subordinate claim. Bonds offer predictability and a stronger position in insolvency, and they pay less for exactly those reasons. Neither is safer in the abstract. They carry different risks.
How investors actually use both
Most portfolios hold both, in a proportion that reflects when the money is needed. Money required within a few years generally does not belong in shares, because you cannot control what the market will offer when you need to sell. Money with a long horizon can tolerate equity volatility in exchange for higher expected returns.
The question is not which instrument is better. It is which risk you can afford to carry, over the period you can afford to carry it.
This is education, not advice
This guide explains general concepts. It does not take account of your objectives or financial circumstances and is not a recommendation to buy or sell anything. If you are unsure, seek independent advice.