You should find out what the minimum holding period is. Now, consider what might happen if you buy the stock at a discount, hold it for a period of time and then sell it at a loss. In the event that you hold the shares for more than two years and then sell at a loss, you don't face this small penalty. Also be aware that while you can buy the stock free of commissions, you will pay a commission to sell the stock. Zero-coupon bonds are bonds that don't pay explicit interest during the term of the loan.
Bonds typically carry interest payments that investors seeking current investment income enjoy.
Instead, zero-coupon bonds provide a return because you can purchase them at a discounted price relative to the principal value paid at maturity. Thus, the interest is implicit in the discount. Calculating this implicit and therefore taxable interest can be a headache, which is why I recommend that you generally avoid buying such bonds in nonretirement-type accounts.