Dependency theory or dependencia theory is a body of social science theories predicated on the notion that resources flow from a "periphery" of poor and underdeveloped states to a "core" of wealthy states, enriching the latter at the expense of the former. It is a central contention of dependency theory that poor states are impoverished and rich ones enriched by the way poor states are integrated into the "world system."

The theory arose around 1970 as a reaction to modernisation theory, an earlier theory of development which held that all societies progress through similar stages of development, that today's underdeveloped areas are thus in a similar situation to that of today's developed areas at some time in the past, and that therefore the task in helping the underdeveloped areas out of poverty is to accelerate them along this supposed common path of development, by various means such as investment, technology transfers, and closer integration into the world market. Dependency theory rejected this view, arguing that underdeveloped countries are not merely primitive versions of developed countries, but have unique features and structures of their own; and, importantly, are in the situation of being the weaker members in a world market economy, whereas the developed nations were never in an analogous position; they never had to exist in relation to a bloc of more powerful and economically advanced countries than themselves. Dependency theorists argued, in opposition to free market economists and modernization theorists, that underdeveloped countries needed to reduce their connectedness with the world market so that they can pursue a path more in keeping with their own needs, less dictated by external pressures.

Basics
The premises of dependency theory are that:

1. Poor nations provide natural resources, cheap labour, a destination for obsolete technology, and markets for developed nations, without which the latter could not have the standard of living they enjoy.
2. Wealthy nations actively perpetuate a state of dependence by various means. This influence may be multifaceted, involving economics, media control, politics, banking and finance, education, culture, sport, and all aspects of human resource development (including recruitment and training of workers).
3. Wealthy nations actively counter attempts by dependent nations to resist their influences by means of economic sanctions and/or the use of military force.

Dependency theory states that the poverty of the countries in the periphery is not because they are not integrated into the world system, or not 'fully' integrated as is often argued by free market economists, but because of how they are integrated into the system.

source: http://en.wikipedia.org/wiki/Dependency_theory