Answer by John Murdoch
Self-Employment Tax is similar to the employer's portion of Social Security and Medicare taxes. If you were a cubicle-dwelling wage slave you would have SSI and Medicare deducted from your paycheck--and your employer would be taxed an equal amount. Since you're the employer and the employee, you pay both halves.
For more information, look here:
If this is news to you, and you have been filing as self-employed for the past five years, you should most definitely see a tax professional.