http://www.fao.org/docrep/005/y4307e/y4307e05.htm
In broad terms, land tenure rights are often classified according to whether they are “formal” or “informal”. There can be perceptual problems with this approach because, for example, some so-called informal rights may, in practice, be quite formal and secure in their own context. Despite these perceptual problems, the classification of formal and informal tenure can sometimes provide the basis for useful analysis. - FAO