Sustainable development is a trendy term, sometimes used simply for communication purposes. The concept itself, however, is clear. The definition proposed by Norwegian Gro Harlem Brundtland, at UNO, in 1987, is: "Sustainable development is development that meets the needs of the present, ensuring fair distribution of the benefits reaped from growth, without compromising the ability of future generations to meet their own needs". Brundtland Report, "Our Common Future".
For the economic progress of our societies, the concept of sustainable development adds two aspects: social equality and preservation of our environment.
On a global scale, the environmental impact translates into reduced biological diversity, depletion of natural resources, increased and globalized pollution of air, water, soils, etc. Consequences are also social in nature. There are growing disparities among different regions of the world - poverty is increasing, child labour is still an issue, literacy is insufficient, etc. Our current mode of development carries its excesses and its limitations. Sustainable development aims to establish a new balance between humanity and the environment.
(Cf. The exhaustive website of Christian Brodhag - Inter-Ministry Delegate for Sustainable development).