All mutual funds distributions should be reported as income, whether you reinvest or not. Taxable distributions come in two forms, ordinary dividends and capital gains. The distributions of ordinary dividends represent the net earnings of the fund and are paid out periodically to the shareholders. Since these payments are considered to be dividends to you, they must be accounted for accordingly.
Capital Gain Distributions are the net gains of the sales of securities in the fund's portfolio and will be taxed at a different rate than that of ordinary dividends. Yearly, your mutual fund will send you a form, called the 1099-DIV, which will have a detailed breakdown of all of these.