Self-Employment Tax

Self-employment tax (SE tax) is a social security and Medicare tax primarily for individuals who work for themselves. Your payments of SE tax contribute to your coverage under the social security system. Social security coverage provides you with retirement benefits, disability benefits, survivor benefits, and hospital insurance (Medicare) benefits.
Generally, you must pay SE tax and file Schedule SE (Form 1040) if either of the following applies.
1. If your net earnings from self-employment were $400 or more.
2. If you work for a church or a qualified church-controlled organization (other than as a minister or member of a religious order) that elected an exemption from social security and Medicare taxes, you are subject to SE tax if you receive $108.28 or more in wages from the church or organization.
Note: There are special rules and exceptions for aliens, fishing crew members, notary public, state or local government employees, foreign government or international organization employees, etc.

Source: (IRS)