The tax levied on both employers and employees used to fund the Social Security program. Social Security tax is usually collected in the form of payroll tax or self-employment tax. The Social Security tax pays for the retirement and disability benefits received by millions of Americans each year. The funds collected from employees for Social Security are not put into a trust for the individual employee currently paying into the system, but rather are used to pay existing retirees. Also, Social Security tax can refer to the tax on the benefits received from Social Security. In the past, Social Security was tax free, but today if you have other substantial income along with your benefits, you will likely end up paying some tax on them.