Self-employment taxes

From our sister site : http://taxhop.com/script/180-paying-your-income-taxes
After calculating your credits, you'll have to calculate any other taxes you owe. If you're self-employed, you'll have to pay the self-employment tax.

This is your Social Security and Medicare taxes. Since you're self-employed, you'll have to pay both your portion and the employer's portion, for a total of 15.3 percent of your self-employment earnings. Attach Schedule SE to show this tax.

This is paid as part of your income taxes (Form 1040 and estimated quarterly payments).

See also Schedule SE : http://www.irs.gov/instructions/i1040sse/index.html