So there you have it. The poor and elderly.
The 47 percent number presumably refers to the percent of the population who don't pay Federal Income Taxes, which of course is just one kind of tax.
Kevin Roose at NYMag puts the data into words:
But back to the 47 percent. There are two primary ways to pay no (or negative) federal income taxes. The first is to be poor, and the second is to be elderly. In 2011, of the 18.1 percent of American households who paid no federal tax (meaning, no federal income or payroll tax), more than half were elderly, and most of the other half were non-elderly people making below $20,000 a year. The other sliver, roughly one in 20 non-payers, were people who made more than $20,000 in household income.
The reason being poor helps is because, with a combination of tax credits (like the earned income credit and the child credit) and deductions, many people earning under $20,000 a year can zero out their overall rate. The primary reason being elderly helps is that Social Security benefits aren't taxed as income, so if all (or most) of your income comes from your monthly Social Security check, your taxable income is marginal or non-existent.