Frim wikipedia http://en.wikipedia.org/wiki/Social_Security_(United_States) see quote below that answers your question.
The Federal Insurance Contributions Act (FICA) (codified in the Internal Revenue Code) imposes a Social Security withholding tax equal to 6.20% of the gross wage amount, up to but not exceeding the Social Security Wage Base ($97,500 for 2007; $102,000 for 2008; and $106,800 for 2009). The same 6.20% tax is imposed on employers. For each calendar year for which the worker is assessed the FICA contribution, the SSA credits those wages as that year’s covered wages. The income cutoff is adjusted yearly for inflation and other factors.
A separate payroll tax of 1.45% of an employee’s income is paid directly by the employer, and an additional 1.45% deducted from the employee’s paycheck, yielding a total tax rate of 2.90%. There is no maximum limit on this portion of the tax. This portion of the tax is used to fund the Medicare program, which is primarily responsible for providing health benefits to retirees.
What it doesn’t explain is that if you are self employed you pay all of it, your part and the part the employer usually pays. I don’t know how much you make in income, but the line about only being taxed up to a certain amount, people who make over that amount, I think it was $106,800 for 2009, see their take home paychecks go up once the obligation to SS is fufilled each year. So, if you make $160K lets say, you might stop paying SS by September.