The self employment tax generally refers to social security and medicare taxes that are paid by self employed persons. The self employment tax includes social security tax at a rate of 12.4% and medicare tax of 2.9%, for a total of 15.3% (13.3% in 2012) of net self-employment income. In 2013, the social security portion only applies to the first $113, 700. Income beyond that during the year is only subject to the 2.9% medicare tax. The change for 2013 is a medicare tax surcharge of 0.9% on earned income, including net self-employment income, in excess of $200,000 for single and head of household filers, or $250,000 for joint filers.

www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Questions-and-Answers-for-the-Additional-Medicare-Tax