Capital gains

First let’s separate capital gains from dividends. Let’s start with capital gains. If I earn a lot of money, I am in the 35% tax bracket which means that the last dollar I earned is taxed at the 35% rate. Now I save my money and I decide to invest it by buying stock in a company and I hold that stock for a year and a day and then I sell it for a profit. Now I pay 15% tax on the profit so I have paid a total of 50%.The capital gains rate has historically been held low to encourage people to invest. Keep in mind that if I just put the money in the bank and earned interest I would pay 35% tax on the interest.
With regard to dividends a company makes a profit and pays 35% tax on that profit. Now they decide to distribute the profit to the stockholders in the form of dividends. The stockholders now pay tax on the dividends which for rich people is 15%. This is why it is called double taxation. Before the Bush tax cuts capital gains were taxed at 20% and dividends at 35%.
For people like Warren Buffett who get most of their money from return on investment they only pay 15%. The 35% is only on earned income (wages). Same with Romney. He had earned income from speeches but most of his income is unearned or from investments.