FICA is the Federal Insurance Contributions Act and it requires employers to withhold Social Security and Medicare taxes from their employees' pay checks. The employer is then responsible for matching the amount of tax withheld and paying it to the government. Self-employment tax is similar to FICA, which is why it's often called SECA, but it is paid by self-employed individuals rather than by employers. At the core of both FICA and SECA is the fact that they are Social Security and Medicare taxes. The main difference is who pays the tax. With FICA, it's the employer who withholds the tax from the employee's paycheck and pays it to the government. With SECA, it's the self-employed individual who is responsible for calculating and paying the tax.