A corporate bond is a loan to a company. It is essentially a way for a company to raise money by offering investors a fixed level of interest on the amount lent (invested).
The level of interest payable to the investor will largely be governed by the financial strength of the company and therefore the likelihood of full capital repayment at the end of the term of the loan.
The lower the risk of capital loss, the lower the interest rate payable and vice versa.
For example, a large blue-chip company such as British Gas is likely to offer a lower rate of interest than a smaller company quoted on the AIM stock market.
Where the risk of default is higher, these are known as high yield corporate bonds.
Corporate bond funds invest in a basket of corporate bonds to mitigate risk. They are generally considered to be lower risk than equites (but higher risk than deposit accounts) and are ideal for income seekers willing to accept a degree of risk to their capital. The underlying capital value can fall as well as rise, but these movements tend to be less than with equities.