"You can solve the problems of the world in a garden." Geoff Lawton. 
Here I will focus on the concepts of the founder of Permaculture, Tasmanian Bill Mollison (b. 1928), although Permaculture itself, is now an international practice. The term 'Permaculture' originated as a merger between the terms 'permanent' and 'agriculture'. It was intended to provide an alternative to commercial forestry and agricultural practices.  Mollison says, " Agriculture is a destructive system." What we do need is a lot more gardeners. [4(I:7)]
Agriculture (commercial cultivation) is responsible for the loss of soil since tilled land has replaced forests. Forests produce soil and they keep it in place. The removal of soil-holding forests has allowed soil to erode down the rivers and streams, into oceans that no longer produce enough oxygen for the earth's living things. In the United States soil losses from cultivation range from 20 to 400 or 500 tons of soil per acre per year. About half of the earth's total was gone by 1950. [4(I:5)]
Household gardens in the U.S. produce nearly 18% of the food, about the same amount as contributed by commercial agriculture. [4(I:7)] Gardens are not only more productive than agriculture, they are less damaging to the environment. Of course, some gardening strategies are more productive and protective of the environment than others. Defining those strategies is what permaculture is about. Permaculturalists prefer non-cultivation techniques.
Mollison worked for the Australian Division of Wildlife. After 1966, with formal training in Bio-Geography and Social Biology, he taught post-graduate courses in environmental sciences. In 1978 Mollison founded and became Director of the Permaculture Institute in Tasmania, Australia. 
Permaculture grew out of two fundamental observations: (1) the earth is running out of critical resources, and (2) remedial action is going to require a fundamentally different approach to mega-problem solving. Permaculture is both a theory of garden design which would mimic natural ecology, and an innovative educational program.
According to permaculturists by 1950 the earth’s major forests were gone. And, forest remnants too depleted to function as real forests were disappearing as well. Forests profoundly affect the earth’s atmosphere, climate, air quality, fresh water cycles, and soil fertility.
Forests moderate climatic stability, so that the death of forests has led to climatic extremes. It is not just global warming, it is extreme weather. Forests recycle most of the earth's atmospheric oxygen. Forest trees store carbon and release oxygen into the atmosphere. 12,000 billion tons of carbon dioxide (1 carbon: 2 oxygen) are released annually by deforestation. [4(I:4)] This is carbon dioxide that would be in use by forest trees that no longer exist.
Forests take in fresh clean water, store it as ground water, and release clean water into the atmosphere. When the forests are removed, the fresh water table that nourishes tree and plant roots, disappears in favor of salinized water containing bed rock salts. These salts move up toward the surface, causing the death of trees and other plants. The result is 'desertification'. [4(I:5)] 
Big Chestnut Trees (left). The big forest trees are dying. Blights and beetles have moved in to remove whole species of the big trees - the chestnuts, elms, poplars, firs, pines, and maples that once formed the major biomass of the earth's expansive forests. The notion that whole species of trees are dying because of diseases and bugs is called the "phasmid conspiracy". In reality, Mollison says, the bugs and diseases are just cleaning up. The trees are already dead. [4(I:5)]
While governments want more studies, most people believe the problem is beyond their scope. Permaculturists say, "we might as well do something. We will never get anywhere if we don't do anything." [4(I:6)]
Remedial action depends on changing our educational system: " We have to move from a passive to an active thought level." [4(I:7)] Training in Permaculture involves, first, participation in a short term, usually 2 week, Permaculture Design Certificate course (PDC). Then, after two years, of applied work, the student may apply for a Diploma of Permaculture Design. In this way, the original course work designed by Bill Mollison, has proliferated all over the world, through students and teachers trained in permaculture design workshops. , , , also see Permaculture Activist Magazine and Permaculture the Magazine (UK).   DG member, Australian andihazelwood is a recent graduate of the Permaculture Design Certificate course. See also this thread in the Sustainable Alternatives forum.
Blueprint for Gardens
Permaculture is a design theory. Its focus is learning the right combinations of plants (and animals) that will be efficient, and sustainable, and then putting those in place. "It is our job to put the right things in place, and then let her rip." [4(I:10)]. Permaculturalists are not particularly concerned about 'native plants'. In fact, Mollison has said, all plants are native to the earth. A greater interest, rather than whether a plant is "native", is whether the plant/or animal, works within the design.  This type of "ecological" thinking is fundamental to the permaculture movement.
One of the ways Permaculture seeks to mimic natural ecosystems is in its concept of guild relationships. Although guilds are sometimes equated with "companion planting", the concept is actually a much more practical design program. Guilds are beneficial relationships between entities that are on-going. Guilds mimic natural plant communities, and often include domestic animals, such as pigs, ducks, chickens, geese, and even fish, as elements in the design of a system.  Once the system is set up very little human intervention is required.
Chicken Heated Glasshouse 
The Food Forest is one resulting system.
A Food Forest then is comprised of beneficial assemblies of plants, called "guilds" that, consciously chosen to compliment one another, create a synergistic whole . . .. The classic example of this is the Native American "Three sisters" guild wherein corn provides the vertical scaffolding for nitrogen-fixng beans while squash grown along the ground acts as living mulch. A Food Forest uses all seven layers or "stories" of a forest: The tall canopy layer, dwarf varieties below that, then the shrub layer, the floral layer, the rhizosphere (underground tubers and roots) and the oft-neglected vertical layer of vines. . . . the resulting exuberant growth makes it difficult to see the trees for the forest! Claude Genest. 
The way the whole system works is much more important than the elements in the system.
"High Roll Way" Wexford Co. Michigan. A meander loop of the Manistee River was used historically as a log roll and by the aboriginal inhabitants as a defensive overlook.
FOOTNOTES: For more information on sustainable gardening practices, please follow these links.
 Geoff Lawton. The permaculture research institute of Australia. http://www.permaculture.org.au/
 Permaculture. Wikipedia. http://en.wikipedia.org/wiki/Permaculture
 Bill Mollison. CV. http://www.tagari.com/?p=54
 Bill Mollison. 1981. 15 original Bill Mollison Lectures presented at the Permaculture Design Course, Rural Education Center, Wilton, New Hampshire. Published by Yankee Permaculture, Barking Dog Permaculture Center PDB52, Spar Florida. pdf http://www.churchofdeepecology.org/permaculture.htm
[4.I] Bill Mollison. Lecture I. Introduction to Permaculture
 Bill Mollison. 2005. Personal Statement. http://www.tagari.com/?p=56
 Tree Facts. http://www.coloradotrees.org/benefits.htm#water
 Findhorn ecovillage, Scotland. http://www.findhorn.info/programmes/programme189.php
 Scott Pittman. Permaculture Institute. Santa Fe, N.M. http://www.permaculture.org/nm/index.php/site/index/
 Claude Genest. Green Mountain Permaculture. Isle Lamotte, VT. http://www.claudegenest.com/greenmountainpermaculture/index.html
 Permaculture the Magazine UK http://www.permaculture-magazine.co.uk/articles/articles.html
 Permaculture Activist Magazine http://www.permacultureactivist.net/
 More Trees With Less Water. Using trees in drought prone croplands: http://www.worldagroforestrycentre.org/water/ pdf
 ABC TV. Gardening Australia. Fact Sheet: Companions/Guilds. http://www.abc.net.au/gardening/stories/s1063743.htm
 Bill Mollison. Chicken heated Glasshouse. Permaculture Two: Practical Design for Town and Country in Permanent Agriculture. http://www.tagari.com/item.php?itemid=3
 Claude Genest. The Food Forest. Vermont Permaculture. Greenmountain Permaculture.com. LaMott, Vermont. http://www.claudegenest.com/greenmountainpermaculture/projects/tagari.htm
Thanks to: www.dreamstime.com for the royalty free photograph of a tasmanian devil;
To: Dawn Raymer, my sister, for the photo of The High Roll Way in Wexford Co. Michigan---our back yard at home;
To: The American Chestnut Cooperator's Foundation, for the photo of Old American Chestnut Trees. http://www.ppws.vt.edu/griffin/accf.html
DEDICATION: For Darius, who wanted to know.