Other replies got it mostly right. There's a particular tax in the US to pay for Social Security and Medicare. If you're employed, your employer pays a half share of that in payroll tax. If you're self-employed, you have to pay the whole thing— about 15% off the top (up to ~$100,000) before paying standard income tax. That's called the Self-Employment tax.

Self-Employment tax applies even if you wind up not owing any income tax. You get the benefit of being able to deduct business expenses, which employees can't do, but that usually doesn't make up the difference.