Trees are a living organism that require water just as much as any other living being, and so when a tree is cut down it is literally sopping wet through sometimes with a moisture content of over 200% This is the main reason that timber is mostly felled during the winter months when the tree is dormant, and the 'sap is down' - the moisture content of the wood is much lower and so takes much less time to dry or 'season'.
Normal woodworking techniques rely on the wood being dry, or fully 'seasoned' and it can take timber months or even years to fully air dry. The advantage of this is the timber's moisture content will be stabilised and the wood will generally remain dimensionally stable when worked. Normal woodworking techniques are generally cutting across or with the grain, but not following it. This results in nice straight, but weaker pieces.
Green woodworking is simply working with wood that is green or 'unseasoned'.
What we mean by 'green' is wood that has been freshly felled, maybe only days or a few weeks ago and is still very wet.
There are various advantages in working with green wood. The timber is much softer and flexible than seasoned wood, and is therefore much easier to work with (especially with hand tools).
Green wood is relatively easy to split along the grain (to cleft or rive) using hand tools, and so negates the need for large rip-saws. Because cleft wood follows along the grain it is naturally much stronger and more supple than wood that has been cut. Saws are a relatively recent invention. Only three sawmills are recorded for the whole of england in the domes day book.
The disadvantage of working with green wood is that as the wood dries out it shrinks and is liable to crack and warp. This can be used to advantage however by using special green woodworking joints that self tighten as the wood dries resulting in incredibly strong joints, without the need for modern fastening techniques such as screws or nails.
It is fascinating to see a useful working greenwood item, be it a rolling pin or three-legged stool, appear out of a piece of tree using your hands alone and simple tools to craft it. There is an immense satisfaction in the experience of clefting in half a freshly felled log, with nothing more than a couple of wedges, a sledgehammer and an axe.