As a simple definition greenwood is merely "living wood, still retaining its sap, and easy to work with edged tools" but this is only a fraction of the story, in truth it is so much more. For thousands of years wood was worked green from everyday items to huge crucks and tresses for cathdrals not to mention timbers for ships.  Hundreds of greenwood trades evolved and survived up until the beginning of the 20th Century.  Working with greenwood, cleaving and shaping the wood alongs its fibres means that you not only retain the shape of the living wood but that you also retain the natural strength of the wood.  Mike Abbot sums it up very well:

"This material (greenwood) has evolved to withstand the enormous stresses placed upon a tree as it is buffeted by strong winds.  Its strength comes not by being stiff but by having the ability to flex.  The green woodworker aims to harness this supple strength of wood to make products that share the resilience and the character of the living tree.

The single most important process used in green woodwork is an operation known as cleaving. To cleave a log is to prise apart its fibres while the wood is still fresh.  Cleft wood follows the flow of the fibres.

Cleft wood compared to planked wood is like spring-steel compared to cast iron."