What is it?
In short, you are expected to help your hosts with their daily tasks, and for that you will be provided with roof under your head, meals, a way of living with the locals and experiencing the country from non-tourist point of view. Every host is different, but the general rule of thumb is around 5 hours of work per day.
How?
First of all, choose the country you are interested in visiting and find their WWOOF website. Registration for the site of the country you choose usually costs around 30$ and lasts a year. When you register, you will have to complete your profile and start sending requests!
What should I know?
You are required to find out if you need a visa for any country you choose and obtain it on your own. Safety wise, it is always a good idea to find out as much as you can from hosts profile, check what other people have written about their stay and have your own discussion with the person that will be sharing their place with you. You can also book few visits in a row to stay in the country for longer or see how it looks like on other farms. Hosts usually specify the period of time they prefer to have their visitors for.
While wwoof'ing will require some work during your stay, it is definitely the cheapest way to travel the world.