I already answered this question. We have a federal INCOME tax. People who have amassed many millions or billions of dollars of net worth can afford to have essentially ZERO income, and therefore pay little or no income taxes. It's only considered income when you realize it monetarily - i.e., a paycheck, interest income, stocks, etc. When you already own everything you need, you actually have very few expenses outside of basic subsistence, which really doesn't add up to that much if you don't insist on eating $300/oz caviar, lobster tails and chateaubriand three meals a day.
It also stands to reason that many rich people got that way by being frugal and being very careful with their expenditures, so just because you have millions in the bank doesn't mean you have to have an extravagant lifestyle. Bill Gates can get by on an electric bill that's the same as any low-income person's bill. Someone could have a billion dollars in the bank, but have a poverty-level income which pays their essential expenses. Furthermore, a lot of that personal wealth tends to be tied up in corporations - both as a capital investment and as a tax shelter. Mit Romney may be worth millions, but probably has various trusts, private corporations or other legal mechanisms set up to manage his money and expenses, and keep his income in that 15% tax bracket.
Tax shelters are a bit of a shell game, but completely legal, and technically available to everyone, not just the rich. The only thing stopping you or me from leveraging those loopholes is a lack of money to pay the experts to manage our money for us (or the expertise and/or time to do it ourselves). If you were to create an incredibly detailed, precise budget and were able to put every spare penny imaginable into sheltered investments and accounts, you too could live at or below the 15% marginal tax rate. Again, not a privilege limited to the rich - if you want to avail yourself of the same things, either work harder, work smarter, or pay someone to work harder and smarter for you. It doesn't just happen because someone is rich, there's a lot of effort. And somewhat ironically, when a rich person pays an accountant or lawyer to do this kind of work, they're directly or indirectly creating jobs in the process.
Corporate taxes are a different ball of wax. Everyone wants to tax corporations, but fails to see the bigger picture, which is that taxes are, to a corporation, an expense that simply gets passed right back to the consumer. So every item you've ever bought already includes a built-in markup in the price that you will never see listed anywhere, which offsets the corporation's tax liability by some small amount. Taxing corporations is another way of saying "let's have a consumer tax."
