On March 23, 2010, President Obama signed into law the Patient Protection and Affordable Care Act, also referred to as the ACA or Obamacare.
Among its provisions, the ACA includes a new 3.8% tax on net investment income that is applied to individual taxpayers beginning January 1, 2013.
In a nutshell, the 3.8% tax is applied to the lesser of: (a) your net investment income, as defined, and (b) the excess of your “modified adjusted gross income” over an applicable threshold.
On December 4, 2013 Forbes published an excellent summary of the new law, including several sample calculations and a Q&A. You can read the entire article here.