The Social Security tax is one of two taxes all employers are required to withhold under the Federal Insurance Contributions Act (FICA). The other is the Medicare Tax. FICA also mandates an Additional Medicare tax, though only for employees earning more than a set dollar amount.
Self-employed individuals generally must pay self-employment tax (SE tax) as well as income tax. SE tax is a Social Security and Medicare tax primarily for individuals who work for themselves. It is similar to the Social Security and Medicare taxes withheld from the pay of most wage earners. In general, anytime the wording "self-employment tax" is used, it only refers to Social Security and Medicare taxes and not any other tax (like income tax).
The Social Security tax is a percentage of gross wages that most employees, employers and self-employed workers must pay to fund the federal program. Certain groups of taxpayers are exempt from paying social security tax. It is the employer’s obligation to withhold the correct amount of Social Security tax from every paycheck and forward it to the federal government on time. Failure to do so can result in significant penalties.
Only the social security tax has a wage base limit. The wage base limit is the maximum wage that's subject to the tax for that year. Refer to "What's New" in Publication 15 for the current wage limit for social security wages.
Employers should record the name and social security number (SSN) of each employee as they’re shown on the employee’s social security card. If the employee's name is incorrect as shown on the card (i.e. because of marriage or divorce), the employee should request an updated card from the SSA. You should continue to report the employee's wages under the old name until the employee shows you the updated social security card with the corrected name.