In the United States, Social Security is primarily the Old-Age, Survivors, and Disability Insurance (OASDI) federal program. The original Social Security Act(1935) and the current version of the Act, as amended, encompass several social welfare and social insurance programs. Social Security is funded through payroll taxes called Federal Insurance Contributions Act tax (FICA) or Self Employed Contributions Act Tax (SECA). Tax deposits are collected by the Internal Revenue Service (IRS) and are formally entrusted to the Federal Old-Age and Survivors Insurance Trust Fund, the Federal Disability Insurance Trust Fund, the Federal Hospital Insurance Trust Fund, or the Federal Supplementary Medical Insurance Trust Fund which make up the Social Security Trust Funds. With a few exceptions, all salaried income, up to an amount specifically determined by law (see tax rate table below) has an FICA or SECA tax collected on it. All income over said amount is not taxed, for 2015 the maximum amount of taxable earnings is $118,500.

More Info: en.wikipedia.org