Surplus value
Following are useful links, successively increasing in complexity, to explore the term for yourself:
www.marxists.org/glossary/terms/s/u.htm
http://william-king.www.drexel.edu/top/prin/txt/marx/marx4.html
http://www.arasite.org/mxsv.htm
http://www.soilandhealth.org/05steve'sfolder/0501steveswritings/050107surplusvalue.html
http://encyclopedia.thefreedictionary.com/Surplus%20value
http://www.mtholyoke.edu/courses/sgabriel/surplus_labor_defined.htm
http://socserv2.socsci.mcmaster.ca/~econ/ugcm/3ll3/marx/cap1/chap09
