Answer:
A share of Citigroup stock represents a claim on Citigroup's assets that gives the purchaser a share of the corporation.
Depending on whether you are an investor or the corporation, a bond is more or less riskier than a stock.
If you are an investor, buying a bond is safer than buying stock since in a worse case scenario where the company goes bankrupt, bond holders are paid before than stockholders. Also bonds provide fixed periodic payments (coupons) and a final payment of the face of the bond at maturity date.
If you are the corporation, issuing bonds is riskier than issuing stock since you have the obligation of making fixed periodic payments to bondholders (coupons) and must pay the face value at maturity date. On the other hand corporations don't have any legal obligation to pay dividends.