What is the difference between stocks and bonds? Which represents more risk to the company? Why?

Buying a company's stock makes you a part owner of the company's assets and therefore entitled to a share of any earnings after expenses, but also at risk for any losses. Buying a company's bonds makes you a creditor, i.e. you are loaning it money.

Your question "Which represents more risk to the company?" confuses me a little. Normally the relative risks of stocks and bonds are evaluated with respect to the investor who buys them, not the company who issues them. Owning stocks is more risky than owning bonds, because if the company goes bankrupt, bondholders get paid first, while stockholders only get whatever assets are left after debts and other obligations are paid. Even if bankruptcy isn't a concern, stock prices are generally more volatile than bond prices. Stock prices are sensitive to future earnings prospects, whereas bond prices fluctuate mainly in response to changes in interest rates.