Self-employment taxes are what persons who work as independent contractors pay in lieu of the Social Security and Medicare taxes.
An employee only pays HIS/HER half of the Social Security and Medicare tax. The employer pays the other half.
The self-employed person pays BOTH sides of the Social Security and Medicare taxes.
Hence, 12.4% goes to Social Security, which is capped at about $104,000 for 2008. This means that, any amount over $104,000 is NOT subject to Social Security taxes.
The Medicare portion is 2.9%, but there is NO CAP on this part. That means EVERY dollar of the net profit shown on the Schedule is subject to that 2.9% levee.