Sustainability put simply is 'staying power', or how long something can go on without outside influence. Is the level of mankind's consumption of oil sustainable? Probably not, many experts believe we have already used half of the easily available oil. A more sustainable source of energy is the sun, which also causes the winds on the Earth.
The increasing number of consumers interested in sustainability have pushed more and more companies to consider sustainability issues in their manufacturing processes and production. Cutting down 200 year old trees to make boards for a hardwood floor is not nearly as sustainable as a bamboo floor, which only requires three years of growth before it can be harvested again. Lumber companies to a certain degree have been concerned about sustainability longer than many industries. If they had not been replanting what they timbered 50 or 60 years ago they would have no trees to harvest now.
Many products have no sustainability. When you buy a tray of chicken breasts at the supermarket what happens to the styrofoam tray they are on and plastic they are wrapped in? I'm sure there is some use for them, but I'm also certain most of them get put into the trash. Even if just the tray was made out of cardboard it could be vermicomposted (composting with worms), the compost could then be put on a field to grow more grain. And after the grain is harvested it, along with excess worms from the compost bin can be fed to chickens to put back on a cardboard tray to sell in the supermarket. This is a much more sustainable model than the huge factory farms and processing that exists today.
Small farms used to operate at a much more sustainable level than they do now. Modern farms have moved away from this model so are forced to use a large amount of outside products. Cattle on feedlots need to medicated because they are not used to consuming only corn, to say nothing of growth hormones to make the grow faster. Joel Salatin in You Can Farm: The Entrepreneur's Guide to Start & Succeed in a Farming Enterprise
As a whole the Earth needs to operate at a sustainable level, but these practices also to need to be followed at ever level. From the entire world through country governments to the family household we all need to think about how sustainable our actions are.