WWOOF is an international system which brings people together in arrangements of mutual benefit: WWOOFers get a place to stay and their food each day without having to spend cash on their board and lodging, in exchange hosts get a few hours' help each day a WWOOFer stays with them.
The acronym WWOOF stands for "Willing Workers On Organic Farms" or "Weekend Workers On Organic Farms". In Ireland WWOOFers tend to stay with hosts for periods of a week or more (the majority come from other countries) but a WWOOF host I know on the other side of the Irish Sea has city-dwelling WWOOFer friends who take care of her farm from time to time while she grabs a weekend away hitting the fleshpots and they enjoy a welcome break in the countryside.
Another typical motive for WWOOFing is to experience the reality hands-on before deciding whether or not to take the plunge and move permanently out of all the hassles of the urban environment.
WWOOF International specifies a basic WWOOF "deal" but hosts can interpret it quite flexibly - I've had people staying with me who've worked like the clappers, full-time, for a period to accumulate "credit" for use later when they immerse themselves in some other activity in the area without the distraction of a daily stint on the farm.