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Earned income is labeled by the IRS as income earned from work. Any other income is ordinary income. There are also special treatment given to certain types of income such as Social Security benefits, capital gains, etc but these are all ordinary income.
If you look at your pay stub you will see that there are two types of taxes withheld from earned income (Social Security and Medicare) that are not taken against any other type of income. Self employed persons also pay these two taxes but it is labeled as self employment tax.