Income derived from active participation in a trade or business, including wages, salary, tips, commissions and bonuses. This is the opposite of unearned income.
Basically, this is any income a person or company gets for which they have done the work or put through the effort to obtain. If your boss gives you an advance on your next check, this would be considered unearned income because you haven't yet done anything to earn it.
AECF.org - A site advocating further tax credits for low-income families.
Income Distribution - A paper by Ruben Cohen on the dynamics of wages, incomes and earnings.