You also need to complete Form 1040, Schedule SE
(PDF), Self-Employment Tax, and pay self-employment tax on your net earnings from self-employment, if you had net earnings from self-employment of $400 or more.
This is the manner by which self-employed persons pay into the Social Security and Medicare trust funds. Employees pay into the Social Security and Medicare trust funds, as well as income tax withholding, through deductions from their paychecks.
Generally, there is no tax withholding on self-employment income.
You may be subject to the requirement to make quarterly estimated tax payments.
If you did not make estimated tax payments, you may be charged a penalty for underpayment of estimated tax penalty.