Another main reason for what is earned income is that you paid Social Security and Medicare tax on it. Like for a job the employer takes out SS & Medicare from your paychecks. It will be on your W2 boxes 4 & 6.
For self employment you pay the 15.3% SS & Medicare as the Self Employment tax on a gross profit of $400 or more. Self Employment tax will be on Schedule SE. It is in addition to any regular income tax you pay on the income.
