Self-employment tax consists of Social Security and Medicare taxes for individuals who work for themselves. Employees who receive a W-2 only pay half of the total Social Security (6.2%) and Medicare (1.45%) taxes, while their employer is responsible for paying the other half. Self-employed individuals are responsible for paying both portions of the Social Security (12.4%) and Medicare (2.9%) taxes.
If you (1) are self-employed as a sole proprietorship, an independent contractor or freelancer and (2) earn $400 or more, you may need to pay SE tax. This is true even if you are paid in cash and do not receive a 1099-MISC. Keep in mind, you may be able to offset this income if you have qualifying expenses.