Sustainability sounds complicated but it's actually something every child understands: leave your place as nice as you find it. More formally, it is sometimes defined as meeting the needs of the present without compromising the ability of future generations to meet their own needs. Environmental sustainability can often be measured by looking at a societies' "ecological footprint."
Redefining Progress, a major sustainability non-profit, uses "ecological footprint" to compare humanity's natural resource consumption rates (its use of resources and waste disposal) to nature's regenerative capacity. Society is presumed to be operating sustainably if its footprint does not exceed the Earth's natural regenerative capacity. In 2004, the United States consumed approximately twice its natural regenerative capacity and its per capita consumption significantly exceeded that of any other country. Businesses are frequently considered sustainable if they follow a triple bottom line with equal concern for people and planet, as well as for profits.
According to a World Wildlife Fund (WWF) report issued this week (story), people are stripping nature at an unprecedented rate and will need two planets' worth of natural resources every year by 2050 on current trends. According to WWF Director-General James Leape, "If everyone around the world lived as those in America, we would need five planets to support us."
See: Act Locally: Ten Steps Toward Sustainability -- Step 1