15.3 is the combined portions of both the social security tax and medicare tax, of which employees pay half (7.65%) and the employers pay the other half. In the case of a self employeed person, they are required to pay both portions...but you can deduct half the 15.3% (7.65%) at years end on your taxes.
If I knew Social Security was going to be there when I retire, I wouldn't mind so much paying that tax. But there is no gaurantee it'll be around in 30 years, and I'd much rather put that 15.3% into my own retirement account.
"You have to pay regular Federal Income Tax and State Tax on what you made (state tax depending on where you live). Also you pay Self-Employment Tax. This is the biggie tax so to speak that you pay. It is 15.3% of business' net income. You do however get to deduct half of that the other adjustments sectino of the tax return."