Source The Dish (per usual)
As we can see, it is true that 47 percent of the population pays no net federal income tax. Many of those people do pay federal payroll taxes on their income, however. Of those who pay neither income nor payroll taxes, most are elderly. That’s because elderly people generally don’t have jobs. Make of this what you will, but in terms of partisan politics it seems very likely that a large share of these elderly freeloaders are actually Romney voters.
Krugman further unpacks the income tax numbers:
[I]f you look at the facts, you learn that the great bulk of those who pay no income tax pay other taxes; also, many of the people in the no-income-tax category are (a) elderly (b) students or (c) having a bad year, having lost a job — that is, they’re people who have paid income taxes in the past and/or will pay income taxes in the future. The idea that half of Americans are just grifters is grotesque.
And there are so many other great explanations for that 47%, keep reading on The Dish.