"The concept of capital has a number of different meanings. It is useful to differentiate between five kinds of capital: financial, natural, produced, human, and social. All are stocks that have the capacity to produce flows of economically desirable outputs. The maintenance of all five kinds of capital is essential for the sustainability of economic development." See the full article, Capital, Encyclopedia of Earth, by Neva R. Goodwin.
This is a good entry level article and provides good links. See also, the Global Development and Environment Institute, Tufts University.