WWOOFing means different things to different people, because it's flexible as to how you go about it.
The basic premise is that you work on an organic farm, usually about 5 or 6 hours a day for 5 days a week, in return for free board at the farm. By joining WWOOF for a small annual subscription you get access to a list of all the WWOOF hosts in your chosen country (you can WWOOF pretty much all over the world, but you need to pay a separate subscription for each country) and their contact details. It is then up to you to contact the hosts direct to arrange when you can go to stay.
You could stay at one farm for a long period of time (months, possibly up to a year at some) or just for a week or weekend before moving on to another farm. Hosts generally specify if they prefer short or long term WWOOFers. Many people WWOOF as an inexpensive way to travel around the country or the world, and a great way to meet people.
Alternatively, there's no need to travel: WWOOF at a local farm for a few hours a week on a regular basis for some fresh air exercise!
Experience is not always necessary, enthusiasm is appreciated more!
WWOOF for more information and to join (there's a brief listing of hosts in the various countries here to give you an idea, but you need to join to get their contact details).