All I can tell you is that what Gov. Romney pays taxes on is NOT income. These are strictly yields from investments. He paid income tax on the money when he first made it.
Investment income is usually taxable. Investmernt income is usually interest, stock dividends, and interest from bonds. (note however that there are things called "municipal bonds" the income from which may be tax free). Mr Romney may have earned income: Taxable. Business income: only the profit is taxable. Capital gains from selling assets at a profit: taxable. All of the above can be very complex transactions that may have separate rules about taxation of them)
Yeilds on investments are income (dividends and capital gains). They are just not wages. They are taxed at a lower rate than wages. Dividends are taxed at a lower rate because it’s a double taxation. Coporations are taxed when they make a profit, then if those profits are distributed to the owners (shareholders), it’s taxed again.