What does 'turned green' mean?
In a nutshell, it means the wood is freshly-cut or wet when it is turned on the lathe...
...but it's a little more complicated than that.
There are two types of moisture in wood: free water and bound water.
If you were to cut down a tree and place your hand on the bare wood of the freshly-exposed log surface, it will feel cold and damp and your palm may even get slightly wet. That's free water and will start to evaporate pretty much straight away as it leaches out of the timber.
After a period of time you won't be able to feel the dampness any more and you might think that the wood is now dry, but you'd be wrong because the cells of the wood contain bound water - moisture bound inside the cells as an integral part of their structure. This moisture won't evaporate as quickly as free water and will literally take years to make its way to the exposed surfaces so it can evaporate.
In fact the general rule of thumb is one year for every inch of wood thickness, providing the wood is stored in a dry environment.
So while the wood contains this moisture it is considered 'unseasoned' or 'green'.
Turning green wood can have its advantages - it cuts easier and is less wearing on the tools; rough-turning a bowl while green means there is less moisture to evaporate when it's finished, which means it dries faster and can be re-turned to its final shape in just a few months; a green-turned item will probably warp and crack as it dries, lending the piece amazing character and oodles of touchy-feelyness. This is caused by the wood fibres becoming stressed as the moisture leaves them, eventually causing them to pull and push against each other and creating unpredictable, often amazing, results.
There are, however, some disadvantages.
For a start, spinning green wood at a million miles per hour makes a hell of a mess as water and sap gets thrown everywhere as well as damp, gluey shavings which stick to everything in sight, including the turner; green wood with a high tannin content, such as oak, turns everything black - tools, walls, ceilings, hands, pets, inquisitive family members etc; and the finished product will probably warp and crack, ruining everything if that wasn't part of the plan.
So whether or not to turn green wood depends on a few factors. The main one of these is patience because timber can be turned immediately after being felled, making it an attractive route to take for the chronically impatient.
Another reason is cost. Wood that has been properly prepared and left to dry naturally needs a lot more processing as well as storage and time, so can be expensive for a turner to buy. It's not unusual for a single dry bowl 'blank' to cost upwards of £20 or more before it gets anywhere near a lathe...
The same applies to kiln-dried wood which is exposed to heat in order to to dry it faster. Again, this makes it more expensive because in addition to processing, storage and time (although not quite so much of the latter), there's also equipment and energy costs to take into account.
...yet green wood is generally free.
Woodturners can often be seen making friends with tree surgeons or council workmen and asking to take a few pieces of freshly-felled wood away so they can work their magic on it. Storm-damaged trees are also good targets (if permission is given by the owner, because all trees - vertical or horizontal - belong to somebody) and even online marketplaces are a great source of free green wood as homeowners just look to get rid of it.
So there you have it.
Now, if the subject ever comes up in a pub quiz you know what it's all about.